How old is she? It can be alot more difficult to get the temps right in a small tank like a 20 gallon. If she's an adult and it won't take long to get her a bigger tank then leave it as it is. As long as she's pooping regularly and you're giving her baths alot (I have 5 adult males & they gets baths every day, but they poop in the water & getting baths daily helps to keep them
hydrated) at least 3x per week, she'll be fine until she's in a bigger tank. She does sound like a miracle & I'm so glad you took her in. Are you giving her good UVB, too? Reptisun 10.0 or Arcadia 12% are the best tube UVB lights & she does need that to digest properly. If you're looking for food choices check out
http://www.beautifuldragons.com under Nutrition, it's the best food list for dragons I've ever seen and I've been raising dragons since 2006.
Welcome to the world of beardies!