Class C Gas Mileage

I have a Class C 1997 Ford F350 V10 Triton 6.8L, that only gets 6 mpg on a good day. No matter if I tow a car or not. It is 32' long. New air filter, oil change, and the brakes are not sticking. Use Sea Foam treatment is gas tank. Any ideas why the mileage is so bad?