The RV park offered you a camp site for a specific price and you agreed to the price and paid it in full. You and the park have a contract. By the RV park raising its price and demanding additional money from you is in violation of the original contract. The additional price was not part of the original contract and was not agreed to by you. You can bring a civil suit for contract violation against the RV park. Or you can blow it off and cancel the reservation and get your money back and the park can keep screwing over campers with no penalty.