After looking on RV Trader and seeing what is there, a likely asking price for this unit would be 25-28K. Certainly things like tires, front ends, batteries are all suspect, and you should know what is right and how old things really are and not just assume they are right.
While you're being noble in your mind, if you took the unit to a dealer and asked what they would give you for it, cash deal, no trade, that should be pretty close to the price you pay too.
If it really was that easy to get more at the dealer, then they should take it.