My dogs skin problem was solved
I am not sure what is going on with your dog but I can tell you that I have a blue Weimaraner and at about 3 years old she developed horrible skin problems which included sores on the bottoms of her feet that would bleed. We tried everything that you have tried and we kept looking for answers. We finally found a Veterinary Dermatologist that was an expert with gun blue dogs. We found that she had an immune deficiency to a skin staph infection. We have been giving her a drug called Staph Lysate for a year, an injection that started twice a week and is now down to once a month. We are told that at the end of this round of treatment that it is likely that she will be cured and that we will not need to give her anymore injections.
All I can say is keep looking for the answer and test for Staph skin problems