A rental site in an RV park is no different than a rental apartment or rental garage or a rental storage area. Give them the address (including lot number if available) but not the park name and don't offer any details unless asked. Just say "this is where we have the RV at this time". It's not fraudulent, assuming you really are in the RV and keeping it on that site/lot at that address. Using an address where you do not have the RV is fraudulent, however, as is any outright lie on the policy application. In theory you should advise the insured when you move the RV to a different "principal garage" location, but in practice few people do. Forgetting to notify the insurer of the new location is an error of omission, not fraud.
Now that I think of it, you could rent a storage site and use that address too. As long as it is the valid location at the time, you are legit. Many people keep RVs at locations other than their fixed home. Note, however, you you also need a billing address if the RV location is different than where you receive your mail and other legal contacts. Again, there is nothing wrong about that and many people, Rvers and others, do it all the time.