I don't really know about any regs involving this, but I will tell you that I camphost at a state park here in Virginia and there is one site that is listed on the website as a handicap site. It is paved while the rest of the sites are pea gravel and gravel; and it has a handicapped fire pit, higher off the ground. At the site, there is nothing at all to designate it as a handicapped site, so anyone that arrives and finds it empty can take that spot.
It is just a guess, but I think they must have one designated as a handicap site, but they don't have to enforce it or even mark it. As I said, that's just a guess.
I have asked the rangers about it, but was told that it was available for anyone to use.