Bev, this is a long shot, but having just gone through this with my mom's dog I'll throw it out her.
If Zisso has not had a stim test to check for Addison Disease, please ask for that. It is a very undiagnosed disease because vets tend to go for treating the gut or stating it is IBD.
Mom's dog has had gut problems for years, with episodes occurring every couple of months. Recently it was growing closer.
She had diarrhea, vomiting, loss of appetite and weakness as well as behavioral changes. She went from a happy dog to a fearful, hiding dog.
Her blood work always came back perfect, right down the middle except low glucose and low electolytes.
Finally last week she had an after hours episode with bright red (fresh) blood in her vomit and her stool. Her gums were white and she had lost more weight. The vet sad she had a nagging suspicion she was seeing addisons.
We treated the symptoms and scheduled the stim test. Normal cortisol levels should fall between 6 and 12. Here initial draw was 0.5. They gave her an injection which should have put her in the normal range, but her second draw was only 2.6.
Definitely Addison. She got an injection and tapering oral dose of steroids.
She was a different dog by the time we got home. For the first time in months she followed me around and even jumped into my lap.
That was Thursday, today is Saturday and she's eating well and very active. She will continue to get a monthly injection for treatment.
Sorry the post is so long, but you might want to consider the test. Addison disease is the opposite of cushings that is more often tested and diagnosed. Most often it isn't considered unless the dog comes in flat out, in coma.
Good luck whatever is going on with Z.
With approval of my friends:
Seiko, Solo, Sophie, Skye and Buddy - bridge dogs who taught me so much