nada prices
When I was looking at the NADA book, I noticed what seemed like a reliable increase between Wholesale, Low Retail and Average Retail (don't remember what they were, though) Get a glance at the book at the library or your bank, and do the math. Should then be able to figure out the wholesale from the retail values given online.
Generally, 75% of list is considered a 'good' deal on a new unit. Make sure you verify what list really is, as there are some dealers who will 'fudge' this value to make a deal seem better. When I first saw my 5th wheel, the 'list' was about $18000. When I saw the 'same' trailer a year later, the list was quoted at $22000. Could have even been the exact same trailer. I think they were quoting the list for a 2002 trailer even though the one they were trying to sell was a 2001...
The dealer is going to attempt to get your unit for less than wholesale, or sell you the new unit at a higher price than you need to pay, or finance you at a profit. Or a combination of these
You need to know the 'actual value' of both units.
For instance, based on the numbers you provided, I would be trying to upgrade for around $45,000. Which means I would start the bargaining around $30,000. This would be most practical if you had the cash in hand. Having a pre-approved loan in your back pocket may be nearly as good. Compare the financing they are offering with your own to see how good a deal it really is. I didn't do my homework on the motorhome, and ended up with a 20 year loan which I find is rather long for a RV.