Post an ad here and I would also suggest CraigsList both here in the SF Bay Area (the original one and largest) and also in Portland and Seattle. Go to:
WWW.craigslist.com
Ultimately, people buying Airstreams are very price savvy. You may be selling it for 20% off list, but that is about what many pay for their new Airstreams after discounts. There is a dealer in Spokane that you could check price with so you can see where you need to be on price. Also, put a full description and a pics to help generate enthusiasm. They are very cool looking and a pic or two in a scenic location would help, as well as a couple of the interior, perhaps with no flash and the interior lights on to create more of an enticing mood. I notice there are a couple others available right now so it may be a bit of a buyer's market at the moment, but many people want those, so you should have no problem once the price is right.
How much are you asking and what are the details?
john