Dealer is asking way too much, you should be paying low retail or less in that condition. You have the right approach- get a professional inspection. Based on your description, I suspect this coach hasn't been used lately with batteries that old. Machines don't like to be idle, so expect problems, even after the inspection. If it has ANY water damage, walk away. Budget $5K for stuff to fix after the purchase if you're handy and can do the work yourself. I'd negotiate the price down and fix things yourself as you will know it's done right. They'll fix it the cheapest they can.
Walt & Bonnie
2006 Monaco Camelot 40PAQ, Cummins 400 ISL
'16 Ford C-Max, Roadmaster BP, Sterling towbar