A few years ago, we did have a paid reservation at a camp site (Travel Trailer). When we arrived someone parked their car in our spot. Quite obvious someone visiting another camper. We couldn't find anyone around who knew who's car it was. After waiting in the road for about 45 minutes, going from campsite to campsite, we had no choice but to go back to the gate house and raise our concerns. Park staff came to the camp site and couldn't find anyone either who owned the car. Finally the campground management called the local sheriff who came out.
Once the sheriff came out and started walking around the car (and we were still in the middle of the road with our camper), someone came running over real fast and moved the car.
At that point, I didn't care what the sheriff or the campground manager did with the guy. I was just glad to get my campsite. The campground was booked, and we were a considerable distance from home! (and we had paid for the site). It only happened once. I suppose it's not so much a problem if you have a trailer that separates from tow vehicle and you can leave it in place.