Since you are buying from an owner, I'm assuming he is not going to accept a trade. ASs for a value, the low retail figure is probably closer than anything. NADA estimates the figures based on a formula of so much off of MSRP, and not actual sales.
Typical? No. A rough rule of thumb use to be a percentage off of low retail, but the bottom line is whatever the dealer has set aside for overvaluing your travel trailer. Of course, the added amount is just added to the purchase price of your new RV. It is the easiest and simplest way of moving up, but you're going to pay for the privilege.
You are so much further ahead selling your trailer yourself, such as on Craig's List. Clean it up like it had never been used and price it right. Search the internet to see what others are trying to sell their travel trailers for and price your unit accordingly. You'll sell it, but it has to be priced right.
Holiday Rambler Navigator DP, Hummer, and Honda VT1100C Shadow