I'm not a vet but the first thing I thought when I saw the picture was "mange". That's a parasitic infection of tiny mites. Have you discussed the possibility of mange with your vet?
The vet can easily see if it is, or is not, mange by taking a skin scraping and looking at it under a microscope. Do you remember any of the vets doing this?
The vet can easily see if it is, or is not, mange by taking a skin scraping and looking at it under a microscope. Do you remember any of the vets doing this?