What do you mean by extra mass on flat?
Can you be a little more specific and clarify what you said? Are you saying that without a load, he gets 12-14MPG while without a load, he gets 11/12MPG?
I'm in a hurry at the moment, but it would be necessary to know exactly how much a truck weighs by itself. A situation with a semi would be more complicated because the aerodynamics are, I assume, considerably worse than a passenger vehicle which would cause wind resistance to be a greater factor.