recurring itchy dog rashes

Question
Hello,
I have a 2 1/2 year old rat terrier mix whom I adopted from a rescue organization as a puppy. Since we got him, he has had pneumonia, demodex, and panosteitis. Now he keeps getting rashes which result in an infection.  We have been to 3 different vets in the past year about this.  Scrapings for mites have come back negative.  They all claim that it is allergies, and prescribe antibiotics and antihistamines.  He has been eating the same flavor of limited ingredient dog food for about a year now. He has been on flea and heartworm meds since he was old enough.
Our last vet said that he did have a severe flea allergy because of the affected area at that time (rear torso and tail), and it took her five minutes of combing to find a single flea. I think we nipped it in the bud, treating all the animals, the house, and the yard.
But now he has a rash on his lower belly/hind leg region. His inner thighs are white and flaky. His upper thighs are bumpy.  While his mid- to lower belly almost look like chicken pox, round scabs.
Benadryl makes him sleepy, which I guess helps him to not scratch as much.  I hate to keep giving him antihistamines and antibiotics.  I'm scared to try steroids because of his medical history-- it seems that he may already have a low immune system.
Any help or advise would be greatly appreciated.

Answer
I agree with the possibility of allergies.  Thyere is a drug called ATOPICA (cyclosporine) that has the same effects on the allergies as steroids, but not the side effects.  Many dogs with bacterial skin infections need antibiotics for long periods of time.  We will use them until the skin infection clears up, then we will prescribe one antibiotic per day for months.