I ran a small business for most of my adult life and what ever deal I made with a customer I stuck to and if I miss-calculated the numbers well that was my fault/my loss! I appreciate the previous posts calling for a moral approach but I have a real hard time seeing the words "honesty and integrity" anywhere near rv dealerships, the guy pointed out the mistake to the dealer and was told it was fine, I'd say the fault was with the incompetent person who wrote it up and sent our friend on his way and here he is up $9.000 sheckles on a coach that was probably well marked up to begin with! How be we all remember that famous old proverb "Caveat Emptor" or "Let the buyer beware" how many rv'ers do we know who got royally screwed but good by an rv dealer, I've met several so in this case the china-bomb is on their rig for a change, I'd say lawyer the heck up and tell the dealer to "suck it up buttercup" and quit whining and in the future pay closer attention to the paperwork!!
Good luck and keep us posted!