It may be that if your dog is on medication to lower cortisol levels, that might be having an effect on her appetite? Usually a dog with raging Cushing's cannot eat enough and has an increased appetite. But medications may be affecting her. Could it be that do you think?
I'm sorry, I don't have personal experience of a dog with Cushing's. I am hoping someone else will post here with more experience to help you.
The only thing I can think of to tempt her to eat is "human food"...that is healthy and non-toxic food that dogs can eat, such as potatoes either boiled, mashed or fried in a little olive oil, chicken, and other meats simmered with vegetables, omelettes....anything fairly bland yet tasty which is not normal dog food. The above "human foods" are perfectly okay for dogs. Dogs can eat fish, canned Tuna and sardines, salmon etc, eggs, starchy vegetables, green vegetables of all kinds, and a little healthy oil in the cooking now and then will do them no harm so long as it isn't overdone.
But what you describe is concerning. It's like she's lost her interest in everything. Yet her tests come back "normal" or not too bad. I wish I could say more to help her and you.
All I can suggest (in the absence of any better medical knowledge about Cushing's) is to try different foods to tempt her.
Have you tried Fresh Pet dog food? I've had some success with this. It's at Petco in a refrigerated case. Walmart has it too. I agree with ginger899. The trilostane medication could be dampening her appetite.