There is no ADA requirement or regulation regarding RV sites. When the ADA mandates something, if the requirements are not spelled out, the standard for compliance is almost universally "reasonable". Don't think you are ever going to find a Judge or Jury who is going to enforce a penalty against a business for not providing a handicapped parking spot (Or any parking spot for that matter) large enough to park a Prevost towing a 30 foot car hauler. It would be my semi-educated guess that if you are handicapped and traveling in an RV larger than a Class B you are going to have to accept the fact there isn't going to be handicapped parking spaces available for you.