Oh, do I feel for you. I have an IBD dog, and another (a broodie, no less) with recurrent UTIs, food issues, and a sensitivity to antibiotics (they make her sick). The broodie is oh-so-slowly coming back from a weight loss similar to Happy's--and she started out too thin.
I agree with Batmom that it's wise to give Happy something for her stomach. Between the possible IBD, the pred, the UTI, and the antibiotics, it's no wonder she's not eating.
That is very important information that you need to keep in mind as you sort through all the advice you're given. Tilapia worked for RobinM's Beau, but it would have killed my IBD dog. Some dogs can't eat beef; I thought my IBD dog couldn't but she can. The broodie is currently surviving on cooked ground beef. There is a lot of interest in grain-free food these days, but most of my girls can eat grains--just not the same ones. One option is to do blood tests for allergies. A lot of people don't like them, but they've worked for me.
Other protein sources? Pork, lamb, duck...all of which are available in kibble, which you might be able to use when you've found a protein source that she can tolerate. There are also even more specialized items (rabbit, kangaroo) available in veterinary diets.
I hope you can find something to make Happy happy again soon.