Consult a Veterinary Internal Medicine Specialist. If you have not seen improvement within a reasonable amount of time from treatment by the regular vet, I think it’s time to step it up a notch. Don’t you? In fact, why not ask your vet for a referral, from what you described, food is not going to fix this. Further testing is needed to diagnose the dog, preferably by a specialist.
Do you even have a diagnosis? Thought so. That is the first thing you want, 3.5 years and no diagnosis? There is no cheap way out of this, but, once the dog starts treatment, hopefully you will have a smooth run.
PS: There is nothing wrong with prescription food. Bloating and gas are symptoms, not a diagnosis.
