CarFax works best when the vehicle's owner uses the dealer's service center (and that dealer sells the brand they are servicing for the customer). Those guys want to be using CarFax to sell their used inventory, so they'll go to the trouble to make sure the service team is reporting everything.
And some premium brands (like Lexus) will act as that conduit between gathering the information and forwarding it to CarFax.
With RV's there is almost no incentive for a shop to report back to CarFax, so that's probably why you aren't seeing any reports.
