My old male Westie has/had skin problems for about 4 few years. He is allergic to fleas...one bite and he's on a tangent to tear his skin off. (Benadryl seems to help some.)
However, he also had a yeast infection that we treated with everything...just like you did with your dog....shampoos, steroids, antibiotics... all of it and nothing really helped. We changed to a prey diet also. It did seem to make a difference in his overall well-being, but it's expensive and labor intensive. He's now on a grain free premium diet with lots of fresh meat, goat milk, whey, yogurt and eggs thrown in for good measure. The thing that made the biggest difference in him was....(this is gonna sound weird)...women's anti-yeast cream, like Monistat. We also tried athlete's foot cream too. I'm not sure which one worked the best for him but something helped him turn the corner. I would bathe him a couple times a week and absolutely slather him in the cream. He hated it! Then I would put a baby t-shirt on him to keep him from getting too sticky and also to keep him from licking it off. After months of trying all the other stuff he slowly started getting his hair back. I don't think he'll EVER get it back on his belly, it looks like elephant skin there, but he has hair on his back!
He was also really, really stinky too and that isn't the case anymore. He hated the ACV too...