Ringworm can be a pain to diagnose. There are MANY varieties of it and any 'test' will only test for certain kinds.
How old is this dog? Mites typically don't like the ears ... ear mites IN the ears yes, but demodex mites rarely affect the ears and sarcoptic mites are going to result in massive massive scratching.
But as Jennie says there are a zillion different type of fungal infections (and ringworm IS a fungus). Has the vet simply tried antifungal creams? You could try tea tree oil/lavender oil - those are both good anti-fungals and can be mixed together.
Various anti-fungals can work on different things -- that sounds VERY fungal to me. You might have to try a dermatologist however, just trying some topicals might help. I've seen tea tree & lavender clear up amazing things.