Ok, I have a customer that has a C-5500, 2006. He is complaining that he is only getting 10 MPG at all times. Towing or not. The truck has a homemade camper on it. I don't know how much the silly thing weighs but the truck is rated for 19,500 Lbs. What kind of mileage should he expect? Seems like 10 is about Par for the course.