On my truck, the tow/haul mode changes the shift points but also tries to keep the engine rpm around 2000 with moving. On the interstate pulling a tractor or car, the truck will still shift to 6th gear if the engine can stay around 1900 or so...
While accelerating, the truck will rev enough so that when it shifts, the rpm will still be in that 2000 range... Now if you have your foot to the floor, it will pull higher rpms, but I am discussing normal driving.
In all reality, I don't see how using it or not would change your mileage much. Still take x amount of energy to move the load....