Boston Terrier rash of some sort?

I can't get this rash to go away. He's had it for months now. I've taken him to him to the vet over and over again. He said it could be this or that, possibly allergies. He doesn't know what he's allergic to, and I've tried changing different things...like his food, my laundry det, etc, and nothing helps. He gave him Revolution and steroids. It helps for a few days then the rash returns as soon as he finishes the steroids. He gets pretty nasty, and I hate that he isn't his normal cuddly self. I'm going to try a new vet, but if that doesn't work then I don't know what to do. Any ideas?