@Tamme3 All of your expressed concerns sound valid, not paranoid. although it doesn't necessarily have to be outside Airbnb dealing- you can just send a special offer which would include the additional fee for him parking the trailer and staying there in addition to your nightly fee for the 5th wheel. If he had 100% stellar reviews, you might feel differently, but that's not the case (but if his other reviews are good, it could also just be a case of an overly fussy host who has a huge long list of house rules- you could cross-reference the host's reviews of other guests to see if there's a pattern there).
I can understand his preference for being on the same property with his mom, but if there's a campground nearby, there doesn't seem any reason why he couldn't stay there. You could call the campground to see if they are all booked up for those dates, then you'd know for sure.
If there's space at the campground, I'd suggest that to him, saying you really aren't set up for what he's asking- say it will void your insurance or something if you want an easy excuse.