my own observation is that NADA values are flavored by the fact (or so, I'm told) that they are taken principally from state motor vehicle / tax agency reporting of declared 'sale price'.
one year ago, when trying to price a 2002 Safari 25C, NADA had its retail value at about $ 11,000. - clearly an absurdly low price.
when trying to find "comparable sales" of an Airstream, it can be hard!
Craigslist is sometimes helpful. eBay may give you some insight, but it's less useful today as you'll only find "Completed listings" going back 45 days in the Automotive section. I found the classifieds here on Airforums a good place to advertise, having bought and sold two other Airstreams since 2008.
I think that Bluto's suggestion might be the most helpful to you. else, use the seat of your pants, and offer it up for what might even be an unreasonable price - the market will give you it's own response!