I'm so sorry to hear of your strugles with Mush and fustrations with your vet. I have heard not to use any pet store vets though because their in the pockets of the most paying reps. As for Royal Canin, BAD-BAD-BAD! My little bully was on that when we bought him from the breeder and he was raw with diarheah. I switched to Natural Balance, it is grain free has sweet potatoe for the carbohydrate witch also helped firm up his stool and i have been able to switch the protein from Chicken, Bison, Duck, Fish, Veal or lamb so he has variety and won't get bored with the same food. I have been able to switch every few weeks with out any digestion problems since the ingredience is all the same. One other thing i give my bully is NuVet Plus. It's a daily supplement(vitamine) I give to help his immune system against allergies,arthritis, digestive issues etc...I know that every ones bully is different and may not react the same and to each his own this is just what has and is working for us.
Hope your bully gets better soon,
Michelle & NiKlauz