First Tow Haul will raise the shift points, and lock the torque converter in the upper gears. Both of these are good things.
Have no experience with a 6L80 transmission, but you should be able to shift into and out of manual mode at any time. DW's car has manual mode and it will improve mileage slightly, not downshifting on grades.
Had a 04 truck with the 6.0L motor and a NV4500 5 speed. At least then you had to maintain 2500 to 3000 for towing up even a gentle grade. 3000-3500 for any real pull. Never felt the need to rev more for power.
