We took a week long camping trip in our 79 Dodge 440ci Fireball last week. This is a 21ft class C with 77k . 10,400 GVW , Dana 70 with 4.10s . Prior to the trip I installed new cap , rotor , plugs and wires. A deep trany pan and pickup . Checked brake pads. Routed ducting for fresh air from air cleaner snorkel to the cowl like they did for Nascar in '65. We logged around 500 miles with 250 of that on dirt mountain roads. MPG was 6.9 no matter what.