has your vet tried atopica? my dog had terrible skin allergies. he would chew himself raw from spring till frost and it kept getting more severe . we went through prednisone ,antibiotics ,benadryl ,etc. to no avail. finally we tried atopica and after a month we started seeing an improvement. we gave him one pill a day for the first month. when things were going real well we started tapering him down to three times a week. now we are back to every day right now because this is one of his worst allergy periods and he started chewing and scratching,and after three days of pills in a row it calmed it back down again. if you haven"t tried it ask your vet about it and give it a try .we were very skeptical at first but now i would't have him be with out it.
Has your vet looked into a possible fungal infection? If you haven't been able to pinpoint the exact allergy(ies) you'll need to go to a larger practice with a dermatology specialist - probably with a university.