I think your approach using NADA value is pretty valid.
But at the same time you have to figure out what comparable used trailers are selling for and how much it's worth to You.
You could reasonably expect to find a few year old trailer that was stored under cover or roof that has less wear n tear from the weather, and you could find one like new inside but maybe some wear n tear that is non existent in a new unit.
Bottom line sooner or later they'll find a buyer and/or come down enough on the price to sell it. Take a swing at it. It's fall and camping season is basically over so if they're smart enough to figure out they'll likely sit on it until next year they may make you a screaming deal on it.
