Fuel mileage is more a function of frontal area and speed than it is engine RPM.
Taller truck with wider tires = more frontal area = more wind drag = worse mileage.
Speed is even worse because it is squared. For example crusing at 80 requires 30% more power than crusing at 70.
Combine both of these and you will take a truck that out of the box did high teens and you should not be surprised by a truck that will do mid teens.
One more thing. Maximizing mileage will minimize performance. I am a big fan of low gears and would put money on that fact that it has less effect on mileage than people give it credit for.