We sold a 1998 Class B Airstream 190 recently. I'll describe our process for reference. Your situation may vary.
We watched ads on Classifieds section of Air Forums to see if any similar models were selling. That didn't provide much help to us because there were so few listings for similar vans.
We also looked at several other online resources: Craigslist, RV Trader, eBay and performed general internet searches to find similar RVs for sale.
The condition/description of many of the vans we saw for sale left something to be desired. Many times, the seller didn't list all the features, didn't say whether appliances worked or not, didn't describe features correctly, didn't describe problems/issues in sufficient detail, etc. I was surprised how many people said things like "never used the black tank," "never used the oven," and similar phrases to indicate the item had little or no wear. For me that was an instant red flag indicating they didn't know if that appliance or feature even worked!
We noted all those asking prices, gave considerable thought to how our RV was worse, the same or better than those we saw in advertisements. In the few years we owned the van we made several upgrades and made a list of those.
We posted ads on Craigslist and Air Forums Classifieds. We had very little activity from Air Forums Classifieds and lots of activity (and ultimately sold) the van via Craigslist.
I suspect Air Forums Classifieds is better suited to selling trailers so you will be likely to obtain lots of good leads from that resource. Don't overlook Craigslist.
Whatever you do... please post lots of pictures in your ad and be very thorough with your description. Use every opportunity to describe why your RV is better suited than a similar RV the buyer may be considering. Don't lie, but if you've made upgrades or taken care to ensure your RV is in top shape be sure to make that known.
We found Kelly Blue Book, Edmunds and similar online pricing tools to be much too low. I don't know how they obtain their estimates but Airstream prices are not accurate IMO.
Be prepared for low-ball offers. We received several and ignored them - we didn't even respond.
Start your homework and good luck.
Lucius and Danielle
1992 29' Excella Classic
1996 GMC Suburban C2500 7.4L
2005 Chevrolet Suburban K2500 8.1L
Got a cooped-up feeling, gotta get out of town, got those Airstream campin' blues...