There really isn't a single "best" venue. It just depends on how wide of an audience you want to reach and whether or not you want to pay for the advertising.
It's best to put your ad up in multiple places if possible. Craigslist, RVtrader, camping forums like this one, Facebook and Twitter are good places online.
If you want to make physical flyers you can put them up on local ad boards at stores and campgrounds.
Put an ad in your local newspaper as well. I've heard some people still get those delivered to them daily.