Well, a salad is the last thing you need and I love tuna but sometimes find hard to digest. I can only tell you what my doctor told me and that was No red meat, fish and chicken only(not fried) cooked veggies, no fresh fruit or veggies, potatoes or noodles. No dairy, no alcohol and no chocolate. No cola's or caffeine. It is boring but it did help. Acidophalous should not make you worse though it can cause some gas.
I would also suggest another gastroenterologist and get a motility study. The constipation can be causing the pain and bloating. Take a laxative like Dulcolax and see if that relieves it for awhile in the meantime. It won't hurt you. I don't remember what tests you have had. I don't know what a GI dietitian is and what credentials he has.