While NADA is an excellent source for determining fair values on white box stuff, IMO, it's useless for an Airstream. There is no quick and easy reference for Airstream values. Watch this group's classifieds closely, along with RVtrader and Craigslist. If you are diligent, over time you can come to a fair opinion of reasonable pricing. I'd say six months or so of scanning and pondering every day.
With Craigslist keep in mind at least half the ads, especially the newer stuff, are bogus. Only pay attention to the ads where the seller posts more than three pictures and actually leaves a contact phone number. Sure some viable sellers do leave ads with no/few pictures, and no contact numbers, but ignoring those will go a long ways towards filtering out the scammer noise.
Call and talk to sellers that are selling models you might have an interest in, then follow those ads to sell how long it takes to get them sold. Eventually you'll get a feel for it.