If you trade in a camper at a dealer, they are not going to offer you only so much on it. That same trailer will be sold by the dealer with a value that you will think to WAY to much. Somewhere in-between those two figures, is a price that private sales will set. Regardless, the value of the camper is determined ONLY by the price the buyer is willing to pay, NOT the price the seller is listing.
If you are buying from a private sale, make an offer that YOU think the camper is worth. See if they take the bite. If not, start negotiations. You've got nothing to loose. If the seller refuses to meet you at your price, then WALK. There's another camper "out there" somewhere. Eventually that seller WILL lower the price to get rid of it.