number of class A motorhomes in use in the USA

Actual number of registered motorhomes information should be available from each state's department of motor vehicles (I am assuming here, I have never actually looked for this information). There probably are fees involved since you are a business basically looking for advertising. That might also explain why you didn't get a timely response from the other organizations you contacted - try again, but be straight forward as to why you want the information, and again there might be cost involved.