Go on Ebay and search "completed listings" for similar units. That might give you a better idea than NADA.
BUT if the unit is in excellent condition (and do a THOROUGH inspection for rot, leaks, and test all systems) and has good tires etc, it could be worth more to YOU than NADA.
IME most sellers will take somewhat lower than asking price. I would consider around $12,500 as a starting offer and be willing to meet him in the middle, minus considerations for needed repairs. Check the date on the tires, if they are good that is worth about $1K.
