You have to look everywhere. Just because Frank lists his on RV Trader, Joe might list his in the local paper or on the bulletin board at the local grocery store, Bob might use EBay, Harry might use the local consignment lot, Harry put his on Craigs List and Ralph may have traded his in at Camping World when he bought a new one. etc, etc.
When you start looking, narrow it down to one or two models, this will be a real time savings so you don't continue to look at everything getting yourself side tracked.
Do use Craigs List, EBay, RV Traders, Lazydays.com, and once you have settled on a particular model, just type it "XXXXXXXXX 26X for sale" into Google and see what comes up.
Bill