It's not needed.
Hydration is what's needed with any sickness, but especially with antibiotics. The juice small amount of juice is just to encourage them to drink, plus it provides a small amount of nutrients.
Making sure he's eating his salad with some
hydrating items is a good idea too.
Baths are only good for
hydration if the dragon actually drinks the
bath water. Dragons don't absorb water through their skin or their vents.
Does he actually drink out of his water bowel? Unless the tank isn't well ventilated, it probably doesn't contribute to the humidity very much. However, most people here don't use waterbowels in the tank for your main concern, and the fact that most dragons don't even use the water bowls in their enclosures.
Good news on the bloodwork. Can you post it up for us to see? With an RI, bacterial or viral, you'd still expect to see some markers in the bloodwork. Is your vet sure of an RI and not just something behavioral or environmental?
Gaping is normal, especially when basking or recently basking, after eating, etc. The throat pushes are normal too, sometimes they just want to stretch a bit.
RI's 99 times out of 100 will come with some form of lethargy, discharge from the nostrils and/or mouth, labored breathing, whistling or clicking while breathing, and a lack of appetite. So i'm curious what your vets reasoning is... unless there were symptoms you didn't mention in your first post.
-Brandon