You can find the NADA Low and Average Retail numbers at
www.nadaguides.com and click Recreational Vehicles.
If the coach is on consignment or a private sales, the seller may not have a realistic view of the market value. All you can do is make an offer and wait for him to come around to it.
Low volume dealers don't even want to come down to Low Retail, so again, make your offer and stick to it. He may take the bait right away if he is hungry for cash, or he may call you in a few days. If not, move on.
I've heard that high volume discount dealers are actually offering LESS discount than previously. With sales down, there are far fewer sales each month to cover the fixed operating costs, so they have less working room on each transaction.