If you join RVIA as a manufacturer you can get access to the commercial statistics they maintain for members. Most likely, these will be production or sales figure by year, not registrations. Don't expect much in the way of response to questions by e-mail, it is just something that industry associations don't pay much attention to, spontaneous requests for the information the organization exists to sell to members.
Registrations for total number of motorhomes will be hard to put together, as many states do not even have a motorhome category. Mine is registered as a "passenger car." Even fewer would categorize by RVIA A, B, C types, which have not been incorporated into vehicle codes generally, the classification is by the industry for the industry.
Neither sales nor registrations help much with "in use" as a significant proportion are unused for years at a time.
