Think you may be right !!!! We'd try one thing for a few days and he would eat it ok, then he wouldn't so we'd try something else, same thing would happen

. that's when the vet said to leave it for a while, pick it up and try again, eventually he just ate the normal food.
What a lot we do for our furry friends !!!
Tuna fish shouldn't upset her and it smells a bit, might encourage her. Try mixing some in with the scrambled egg?
If she was used to having what her elderly owner had no wonder she's turning her nose up at dog food

You might have a long job on your hands but you'll get there eventually