If the campground is a "seasonal" campground, find out from the management what the campsite value is worth to rent with no camper pre-existing on it. It sounds to me like the campground must use some kind of "leasing" system and the people selling are attempting to get out of the lease and and sell the camper to. Look at both items separate. How much time is left on the lease and what is the price of the original lease minus a pro-rated usage the existing owner has already used (time). Then take the value of the camper and the value of the remaining lease and add the two together.
If the campsite is actual property purchased and the "Owner" (whoever holds the deed) is the seller of the trailer and not campground management, then you're probably best to check with a real-estate agent for comparable values of the real-estate.