I have read that only certain aspects are illegal to discriminate a customer over, one being the person's age. I have stayed at a few 55 and older campgrounds and while that works for me, I suspect that would be against the federal discrimination laws for those under 55.
Also, quite a few years back I reserved a site and was asked how old my RV was and I told them it was brand new which was true. When I arrived they told me sorry, but you have a Truck Camper (Brand new top of the line Lance) and that they don't allow Truck Campers or Popup Campers. I asked why I wasn't told this when I reserved the spot over the phone and I was nice about it, the manager said he was only the manager and didn't make the rules however since I was only going to be there for one night to go ahead and that he hoped no one would complain as he was OK with it but the owners were not and he was concerned if it got found out he could get in trouble. Being nice to the manager likely helped me in this case.
I most often stay in State Parks and National Parks although once in a while private parks as well. State and National Parks don't seem to have any concerns. If a private park did have a concern, I wouldn't care to stay there anyway. ~CA