You can spend a lot of money dinking around. That's weird enough looking that I would go straight to a local dermatologist, if there is one around. Personally it looks like infection to me, like maybe something fungal, but I don't know a whole lot about weird skin things.
I agree that if it were grass, it would be a different pattern. It might be the sun aggravating an infection - it's in the groin area because that stays more moit and stuff can grow.
To me, it's clear your vet doesn't know what it is. My experience is that an experienced specialist can save you a lot of guessing and unnecessary meds. If it's infection, the steroids already were not good for your dog
