I can assure you that most all of their issues are due to inadequate UVB light (prolonged). Without any adequate UVB light they cannot digest anything they eat properly, cannot absorb any nutrition, vitamins, minerals, etc., including calcium, from any of the food they eat or the supplements you're giving them, and they cannot use/process any of it.
I don't think this is a "disease" or illness at all, though it could very well be a bowel impaction or obstruction due to the inadequate lighting. It effects everything their bodies need to do to survive. Without at least 13-14 hours every day of adequate, strong UVB light they cannot live. I forget at this point what UVB lights you have, but if I recall they were very weak and questionable brands, like "Trixie"...no, 13" away from them or further is too far away for the best UVB light you can buy, even the extremely strong T5 UVB tubes must be within at least 11" of them unobstructed, so if you have those very weak UVB tubes any further away than 6" or they are obstructed/sitting on anything like a mesh lid or glass/plastic that is blocking the light, they are still getting zero UVB light.
Even the very good T8 UVB tubes, like Reptisun and Arcadia have to be within 6" of your dragon, at the furthest, so you absolutely must find a way to get those UVB lights you currently have within 6" of them, especially the sick guy.
Also, do you still have a bright white basking light over them all, especially the sick guy, and have a Basking Spot Temperature between 100-105 for him/all of them? The reason I'm asking is because dragons naturally absorb all of their heat from bright white light overhead, not underneath them, and you mentioned putting him in a plastic tub with a heat mat. It's fine to have a heat mat in there for him (make sure he's not laying directly on the mat but rather on a towel wrapped around the mat), but you still need to have BOTH the bright white basking bulb and the UVB light directly over his basking spot/platform, have the temperature of that Basking Spot/Platform between 100-105, even a bit higher while he's so sick, but a maximum temperature of 110, and have the UVB light within at least 6" of him, no further, and if you're using that "Trixie" brand UVB light or the other one (both if I recall are very weak T8 UVB tubes) I'd try to get it at 4"-5" if possible, but within 6" is the furthest away max that the UVB can be to just simply deliver the weakest UVB light from that tube to him.
This is crucial. If he does have an impaction (he can still pass small poops and have a bowel impaction/obstruction, especially if you're feeling a lump), keeping his Basking Spot Surface Temp at least at 105 degrees Fahrenheit is crucial, along with getting that weaker UVB tube within 6" at the furthest from him, because otherwise he will continue to not properly digest his food (which will just continue to add to the impaction), not absorb any nutrition from anything you feed him from neither food nor supplements, and will continue to get worse and worse. So try to find a plastic tub that is not as tall or a Basking Spot/Platform that is taller in order to get that UVB light within 6". And please be sure to have both that UVB light and a bright white basking bulb over him for at least 13 hours every day.
I don't know where you live in the world, but you need to buy an electrolyte replacement drink for him and give it to him by syringe. Whether it be Pedialyte or another electrolyte replacement drink for human babies (that's what Pedialyte is, it's not sold in any pet shops, it's sold in all grocery stores and department stores like Walmart here in the US, and is always located in the baby food aisles. Most all countries are going to sell an electrolyte replacement drink for babies/toddlers, or if not then you shouldn't have any problem finding sugar-free Gatorade, which is essentially the same thing, so get that if you can't find the Pedialyte or equivalent human baby electrolyte replacement drink. Give this to him straight from the syringe several times throughout the day. That vet should have given him a subcutaneous injection of an IV electrolyte solution, like Lactated Ringers, which would have perked him right up.
You can actually buy IV bags of Lactated Ringers solution here in the US without a prescription online or in medical supply stores very cheaply, I had to give my old pug a subcutaneous drip of Lactated Ringers solution twice a day (he had a bad urinary tract infection at age 13 and his kidneys started to shut down, and he displayed symptoms just like your dragon is). I was in graduate school and broke, and had already paid $695 for all of the blood work, x-rays, an ultrasound, and a catheter and urinalysis, so I couldn't afford another $2,000 to keep him in the Animal Hospital for another week so he could get Dialysis. So instead I brought him home and rigged up a "poor man's Dialysis" for him. He was dying and I had no choice, and a little medical training and education at that point, so I bought (4) 1 liter IV bags of Lactated Ringers solution, an IV starter kit with tubing and a drip line, and then a bunch of 18 gauge IV needles to hook to the IV line. I hung the IV bag up above us as I sat on the floor in my living room with him, I attached the IV drip line to the bag (they have a drip valve on them that either opens or closes the IV drip and can speed it up or slow it down), then I attached the 18 gauge IV needle to the drip line. Then I would sit in front of him, squeeze the big wrinkle of skin up on the back of his neck, right above his shoulder blades, and I would insert the big IV needle right under his skin and hold it there. I then would reach up and open up the IV drip all the way, full blast, and I would have to sit there with him, holding this big-ass needle that was inserted under his skin, until half the IV bag of Lactated Ringers solution would flow into him and under his skin. Once half the bag was in (took about 15 minutes a pop, the entire time him sitting there like a champ, not moving at all, while I held the largest bore IV needle I'd ever seen under his skin for 15 minutes). He'd get this huge lump/hump in his back, which was the fluid under his skin. After half the bag flowed in I shut off the drip and pulled the needle out and held pressure on the site with a piece of sterile gauze until it stopped bleeding. It would take about an hour for his body to absorb all of the fluids and the big lump to go down. I did this twice a day, every day for 2 weeks.
He went from losing weight, vomiting, having black diarrhea, being completely lethargic and disoriented, weak in his legs and falling over, blood in his urine, and I believe partially blind, and having the emergency vet at the Animal Hospital tell me to just euthanize him on the spot (I grabbed a prescription from him for amoxicillin, paid the bill, wrapped him up in his blanket, and took him home), to being 500% better the next day, to perfectly fine in 2 weeks. He lived another 2 years and passed away from old age, in his sleep, at the age of almost 16. I got almost 3 more years with him because of simple, inexpensive, subcutaneous fluids.
I'm telling you this story because this is what he needs most right now (in addition to adequate UVB light and temperatures), if he does have some type of infection or parasite, it's causing his kidneys to overload and fail. I also gave Jaeger an Amoxicillin tablet twice a day along with the fluids, but it wasn't the Antibiotic that saved his life, it was the subcutaneous fluids. And even a general vet can give your dragon subcutaneous fluid injections and easily instruct you on how to give them to him daily at home. We've had numerous members on here been given the fluids (Lactated Ringers solution is best in this situation) and the syringes with needles already attached, often the syringes are already filled and they show you how to give him the fluid injections at home, it's easy, right under the skin. I'd be calling around to any vets you can find ASAP and asking about this, he needs fluids/electrolytes daily, and you're not going to be able to get enough in him by mouth to help, nor can you find the correct solution he needs. He cannot wait for this vet to come back for the fluids, you can always take him to that vet when he comes back because he needs blood work done ASAP, but right now he needs subcutaneous fluids immediately to survive, along with the lighting and temperature adjustments I've already mentioned...
I don't think this is a "disease" or illness at all, though it could very well be a bowel impaction or obstruction due to the inadequate lighting. It effects everything their bodies need to do to survive. Without at least 13-14 hours every day of adequate, strong UVB light they cannot live. I forget at this point what UVB lights you have, but if I recall they were very weak and questionable brands, like "Trixie"...no, 13" away from them or further is too far away for the best UVB light you can buy, even the extremely strong T5 UVB tubes must be within at least 11" of them unobstructed, so if you have those very weak UVB tubes any further away than 6" or they are obstructed/sitting on anything like a mesh lid or glass/plastic that is blocking the light, they are still getting zero UVB light.
Even the very good T8 UVB tubes, like Reptisun and Arcadia have to be within 6" of your dragon, at the furthest, so you absolutely must find a way to get those UVB lights you currently have within 6" of them, especially the sick guy.
Also, do you still have a bright white basking light over them all, especially the sick guy, and have a Basking Spot Temperature between 100-105 for him/all of them? The reason I'm asking is because dragons naturally absorb all of their heat from bright white light overhead, not underneath them, and you mentioned putting him in a plastic tub with a heat mat. It's fine to have a heat mat in there for him (make sure he's not laying directly on the mat but rather on a towel wrapped around the mat), but you still need to have BOTH the bright white basking bulb and the UVB light directly over his basking spot/platform, have the temperature of that Basking Spot/Platform between 100-105, even a bit higher while he's so sick, but a maximum temperature of 110, and have the UVB light within at least 6" of him, no further, and if you're using that "Trixie" brand UVB light or the other one (both if I recall are very weak T8 UVB tubes) I'd try to get it at 4"-5" if possible, but within 6" is the furthest away max that the UVB can be to just simply deliver the weakest UVB light from that tube to him.
This is crucial. If he does have an impaction (he can still pass small poops and have a bowel impaction/obstruction, especially if you're feeling a lump), keeping his Basking Spot Surface Temp at least at 105 degrees Fahrenheit is crucial, along with getting that weaker UVB tube within 6" at the furthest from him, because otherwise he will continue to not properly digest his food (which will just continue to add to the impaction), not absorb any nutrition from anything you feed him from neither food nor supplements, and will continue to get worse and worse. So try to find a plastic tub that is not as tall or a Basking Spot/Platform that is taller in order to get that UVB light within 6". And please be sure to have both that UVB light and a bright white basking bulb over him for at least 13 hours every day.
I don't know where you live in the world, but you need to buy an electrolyte replacement drink for him and give it to him by syringe. Whether it be Pedialyte or another electrolyte replacement drink for human babies (that's what Pedialyte is, it's not sold in any pet shops, it's sold in all grocery stores and department stores like Walmart here in the US, and is always located in the baby food aisles. Most all countries are going to sell an electrolyte replacement drink for babies/toddlers, or if not then you shouldn't have any problem finding sugar-free Gatorade, which is essentially the same thing, so get that if you can't find the Pedialyte or equivalent human baby electrolyte replacement drink. Give this to him straight from the syringe several times throughout the day. That vet should have given him a subcutaneous injection of an IV electrolyte solution, like Lactated Ringers, which would have perked him right up.
You can actually buy IV bags of Lactated Ringers solution here in the US without a prescription online or in medical supply stores very cheaply, I had to give my old pug a subcutaneous drip of Lactated Ringers solution twice a day (he had a bad urinary tract infection at age 13 and his kidneys started to shut down, and he displayed symptoms just like your dragon is). I was in graduate school and broke, and had already paid $695 for all of the blood work, x-rays, an ultrasound, and a catheter and urinalysis, so I couldn't afford another $2,000 to keep him in the Animal Hospital for another week so he could get Dialysis. So instead I brought him home and rigged up a "poor man's Dialysis" for him. He was dying and I had no choice, and a little medical training and education at that point, so I bought (4) 1 liter IV bags of Lactated Ringers solution, an IV starter kit with tubing and a drip line, and then a bunch of 18 gauge IV needles to hook to the IV line. I hung the IV bag up above us as I sat on the floor in my living room with him, I attached the IV drip line to the bag (they have a drip valve on them that either opens or closes the IV drip and can speed it up or slow it down), then I attached the 18 gauge IV needle to the drip line. Then I would sit in front of him, squeeze the big wrinkle of skin up on the back of his neck, right above his shoulder blades, and I would insert the big IV needle right under his skin and hold it there. I then would reach up and open up the IV drip all the way, full blast, and I would have to sit there with him, holding this big-ass needle that was inserted under his skin, until half the IV bag of Lactated Ringers solution would flow into him and under his skin. Once half the bag was in (took about 15 minutes a pop, the entire time him sitting there like a champ, not moving at all, while I held the largest bore IV needle I'd ever seen under his skin for 15 minutes). He'd get this huge lump/hump in his back, which was the fluid under his skin. After half the bag flowed in I shut off the drip and pulled the needle out and held pressure on the site with a piece of sterile gauze until it stopped bleeding. It would take about an hour for his body to absorb all of the fluids and the big lump to go down. I did this twice a day, every day for 2 weeks.
He went from losing weight, vomiting, having black diarrhea, being completely lethargic and disoriented, weak in his legs and falling over, blood in his urine, and I believe partially blind, and having the emergency vet at the Animal Hospital tell me to just euthanize him on the spot (I grabbed a prescription from him for amoxicillin, paid the bill, wrapped him up in his blanket, and took him home), to being 500% better the next day, to perfectly fine in 2 weeks. He lived another 2 years and passed away from old age, in his sleep, at the age of almost 16. I got almost 3 more years with him because of simple, inexpensive, subcutaneous fluids.
I'm telling you this story because this is what he needs most right now (in addition to adequate UVB light and temperatures), if he does have some type of infection or parasite, it's causing his kidneys to overload and fail. I also gave Jaeger an Amoxicillin tablet twice a day along with the fluids, but it wasn't the Antibiotic that saved his life, it was the subcutaneous fluids. And even a general vet can give your dragon subcutaneous fluid injections and easily instruct you on how to give them to him daily at home. We've had numerous members on here been given the fluids (Lactated Ringers solution is best in this situation) and the syringes with needles already attached, often the syringes are already filled and they show you how to give him the fluid injections at home, it's easy, right under the skin. I'd be calling around to any vets you can find ASAP and asking about this, he needs fluids/electrolytes daily, and you're not going to be able to get enough in him by mouth to help, nor can you find the correct solution he needs. He cannot wait for this vet to come back for the fluids, you can always take him to that vet when he comes back because he needs blood work done ASAP, but right now he needs subcutaneous fluids immediately to survive, along with the lighting and temperature adjustments I've already mentioned...