That coach should have a exhaust brake, most likely on the left side where they usually install the exhaust brake switch, touch gear control, mirror control and heater switch, control for spotlight.
Sixty five thousand miles for a diesel is just breaking in. The only thing that raises a red flag for me would be that the dealer does not let you test drive the coach. There is something fishy there.
I am not a Cummins person, but some folks like them. I had bad experiences with the ISB engines, but on Dodge pickups. But other folks swear by them. Either way diesel engines are superior to gasoline engines in my opinion.
