I get paid to provide "professional" appraisals for large industrial printing and binding machines frequently. The best ways to establish fair market value are the same with RVs as they are with industrial machines or real estate.
1. If there are any established providers of information like NADA, look at them for reference. Banks who loan money on RVs use wholesale NADA value. That is somewhere around 13% below retail NADA.
2. Comparable sales. See what other people have paid. ebay will show you solds. PPL will show you solds.
3. Check to see what other people are asking.
4. And now the hardest one. You have to use your judgement to sense how the market is. If you sense things are good ask a little more. If bad the opposite. But whatever you do do NOT start at a high price to "test the market" and then think you can come down later. Start at a fair market value and add just a bit for negotiating room.
Country Coach 42' Affinity
2013 Jeep Wrangler Unlimited