There is no set discount on used vehicles. The dealer can ask 50% more than it is worth or may well ask a reasonable price that he does not plan to negotiate on. You are trying to buy a motor home, not a discount.
It is simple enough to look up the NADA value on-line and then search for similar for sale units across the country. What they can actually be purchased for is the true value...certainly not some dreamed up discount percentage.