Believe me, we know how hard this is. As vets, we often get so involved in these kinds of cases that we take it as a personal fault when we cannot fix them and it sound s like you have already done so much. I'm sorry I couldn't offer a lot of new information. I will really try to give you some more help now.
Medications you might try:
Fortiflora or a like probiotic
Budesonide (safer than prednisone)
Fiber (in the form of 100% canned pumpkin or Metamucil): it can sometimes really help with the gas production
Sweet Potato: You might find that this works better than actually potatoes
I feel that raw diets can be useful in many situations as long as they are balanced appropriately. As vets, we need to start to accept some of these alternative treatments and diets...they can be quite valuable.
For nutritionists, you can try the following:
Online Resources (these websites link you to already formed diets or will consult with you for a fee...all are recommended by veterinary nutritionists):
balanceit.com (this site also sells homemade diet additives to get the nutrition balance)
For veterinary nutritionists, I found the following in New Jersey. Unfortunately, I am not too familiar with the state but I think New Brunswick is close???
Sarah Ralston (New Brunswick)
Laura Eirmann (Paramus)
Lisa Weeth (Tinton Falls)
I hope that helps!