Okay..here is my opinion on your situation.
What your cat has is chronic gastritis...obviously. Intermittent vomiting for longer than a few days...
The causes are varied and truthfully your vets, as frustrating as it seems to you because you have spent the money and have no cure, rule out many things with all those tests. People forget that we can only go on what an owner tells us, since a cat doesn't communicate very well. Your vets ruled out some pretty major disease processes that cause chronic vomiting. Unfortunately, it leaves you frustrated.
Now causes of gastritis can be esophageal reflux, inflammation or upper stomach inflammation, and sometimes even low grade pancreatic inflammation. With these things on a low grade chronic issue...you don't see changes in the bloodwork or the xrays...and endoscopy just may show a few red areas...but no diagnosis.
So...what do you do? At this time you treat the symptoms. Maybe what is causing this gets better or is managed...or it gets worse and there are changes in the bloodwork or intestines to actually diagnose. One thing that will get worse is inflammatory bowel disease which will be considered if vomiting continues, weight loss, and even diarrhea...for that they will probably want intestinal biopsy..just so you know.
I will not get into a debate about foods. There are lots out there...your focus should be on the possibility of food allergy playing a possible role. You need an easily digestible food that is moderately fat restricted and some single protein based foods. You need to probably avoid the fiber foods (W/D) because it takes the stomach more acids to break down. If you want specific names of veterinary endorsed foods, let me know.
I would continue the Pepcid. I would have your cat wormed with fenbendazole (ask vet) because there are some strange little worms that we don't see very often that have been linked with chronic gastritis...and for whatever reason...we forget about them. I wouldn't do a fecal, just ask for the wormer. Next ask for a prescription for Reglan syrup. It helps gastric motility so the food doesn't sit there and is actually an anti-nausea medication.
I am thinking your cat has hunger but the minute she wants or goes for food she deals with gastritis or possible tummy discomfort and she just doesn't want to deal with it. If you felt like you were going to vomit...you just start avoiding food. The scary thing is cats will become anorexic out of this habit...and then we get into liver issues.
Now...one thing you can try as well until you can get her back to eating...is plain jar baby foods (without garlic or onion added) of lamb, turkey or chicken. In the clinic we warm so the have a nice "fresh" smell and the cats lick it up and don't have to chew it.
Okay..I think that is it. Let me know if you have questions.