OP, I'm sorry about your girl.
I'm breaking this down into section so it doesn't seem quite so overwhelming.
Testing:
Bloodwork, including thyroid testing, should have been done a long time ago. Two years out with no diagnosis AND no vet insisting on some type of bloodwork is pretty sad.
A skin scraping with cytology should have been one of the first or the first thing done. some things, including mites, are visable on cytology. Even if mites were not seen (hard to catch with skin scraping), some vets will treat for them anyway just in case (with ivermectin or selamectin).
Allergies:
I'm not a vet, so I can't diagnose; but it is my opinion that while there may (r may not) be some allergy involvement, there is something more going on. And it is also my opinion that steroids should have help more than what they did if this was purely allergy related (food or environmental) unless it was a very low dose. Atopica, while I highly doubt it will do much good, could have been given with antiemetics for the first week or two.
Food:
If food allergies are still a concern, an elimination diet will have to be done. As Ann said, chicken, turkey, beef, and lamb will need to be avoided. Also add goat and pork to the list as these are common ingredients in kibbles, so they are not novel to a large number of dogs. It may be best to go grain free during this time. Sweet potato may work well. However, I would still say there is more to this than an allergy (imo). Also, it looks like the vet put her on an rx diet to stop the formation of bladder stones. If other foods are given with an rx diet, it could make the rx diet less effective. In this case, a vet (preferably a derm or internist) needs to decide what is more important: an allergy elimination diet or a diet to stop stone formation. It is also possible that a diet exists or could be created to control both issues at once. For this, you will likely need to talk to a veterinary nutritionist. Sorting this all out is really not something that can wait. They are trained to be able to figure these types of things out. Some table foods, including ham, can be very harmful to dogs and cause some very serious health problems. Please be very careful about what snacks are given.