As with real estate, the only way to determine true value is to see what someone is actually willing to pay for it. Just because a bank will lend X amount, doesn't mean it's worth that.
I say, if you find a trailer you like, and come up with a price you're willing to pay, then that's what it is worth to you. It doesn't matter what a bank will lend or what some guide is saying it's worth if you don't think it's worth that.
With our last trailer, NADA said it was worth $27,000. We were lucky to get $20,000 when we traded it in. Zero bites when trying to sell it ourselves. So, to me, it was worth $20,000 (which sucks considering we paid $34,000 for it 2 years prior). Dealer sold it for around $25,000, not much less than we offered it for when trying to do a private sale. They have the advantage though, because they make it easy to purchase.... if you buy private you have to get financing (assuming you aren't paying cash of course) lined up, etc etc all by yourself. At a dealer, they make it easy, and also I think many people think buying from a dealer vs a private seller is "safer".
I think you'd be better off just trying to find a private seller for a trailer in that price range, than going through a dealer. Or, just offer what you think it's worth at the dealer... they can take it or leave it.