Same thing as the "anti-rust" wax car dealers try to sell. Don't bother. Just wash and wax your camper. If you really want you can spray the interior cushions yourself with something like Scotchguard, but I don't think that is necessary either.
As to buying a full cover to go over your camper, that is up to you. Plenty of reading about that here on these forums. I cover my TT up here in New England because the camper sits for months during the winter. Other people don't. Ideally, you would be able to park the camper in a suitably big garage!