NADA is reflective of past sales that have occurred at dealers. Many banks and credit unions use that value to determine what they will loan. I would expect private sale prices to be somewhat less than dealer sale prices. If you are actively looking, I suggest that you subscribe to NADA RV. You can do it for 30 days for free. Then you will see tha average wholesale/trade in prices that dealers pay as well as average retail.
I just purchased a 2013 Interstate on a 2013 Sprinter for 13k less than the NADA retail price from a non-Airstream dealer. I had a similar price offer for a 2013 AI on a 2012 Sprinter from an Airstream dealer. That was in a color I found less desirable (black) and the unit had 14k more miles on it. I travelled 1200 miles for the silver unit I wanted.
You have to be willing to negotiate and to travel to get the best deal. I had been looking for months, following pricing on rvt.com and RV Trader, as well as Craigslist ads. When I was ready to purchase, I knew what a good price was. The value of knowing the the NADA trade in price with a private seller, is that you have some idea what the seller could get from a dealer, and if you are buying from a dealer, it gives you an idea what the dealer might have into the coach.