but what keeps someone from just hitching your trailer up to their vehicle and leaving the campground?
Ask him how does he keep his car from getting stolen. The obvious answer is hitch locks. Not so obvious is the fact that most campgrounds require guests to check in at the gate and state what site they are going to or who they are there to visit. Vehicles that are camping there must show a pass (usually hung from the rear-view mirror) to get in. While many complain about how much having guest sometimes costs, it's actually a built-in safety net to avoid what your husband expressed concern over. Whether it's a TT, 5th wheel, Class A, B, or C a professional is going to get it if he wants it, but most likely it will be from a parking lot or your own driveway, not a campground.