1st I can believe if you are on flat and open highway with no head winds or side winds and are keeping your speed down then 24 would be possible with cruise control. But if you are pedal driving and any amount of traffic the slight changes made in throttle will each that mileage up. I assume very few of us get to travel any real distance in which there is not some evlivation change greater than 150 - 200 ft at least or where you can lock cruise control at 70 or 72 and not have to change you speed for one reason or another for an extended period. I know that I can get 17-18 at 75+mph heading over the west end of the Mass Pike, if you do not know it, it has some large elevation changes, is only 2 lane in each direction, so traffic plays into it as well.
As for increasing mileage you need to decrease the drag on the truck. Lowering it will help. Additional skid pans / bottom plating will also decrease the drag, as will a sloped back cap on the bed with the gaps between cab and cap filled with weather stripping or similar. As for the steel vs painted bumper, based on visuals and water trace lines I see on these trucks, the painted bumper would be more efficient, particularly as it has that heavy chin splitter and wants to push air around the front fairly smoothly.
However, I am also from the school of thought..If you want fuel mileage then do not buy a truck, cause there is just no way to make a brick still look like a brick and be aerodynamic. The changes to make it truly efficient would make it no longer look like a brick.