It’s kind of like transmissions these days. You normally (always) use “D” but the manual gear options exist, for those one off times you might want them. Point being, just run auto. I wish it came on auto when you pushed tow haul. Oh well. These trucks tow so well

 

Here is what the manual notes:
Non-Auto:
1) The engine braking feature only functions when you release the accelerator.
2) You can shift the transmission to lower gears with progressive range selection (PRS) or manual shifting independent of pressing the brakes.
3) You can also use manual engine braking with cruise control to improve grade descent control
4) If your vehicle has Adaptive Cruise Control, the speed control system uses the selected engine brake mode, transmission gears, and brakes to maintain the set speed and distance to the vehicle being followed.

Automatic Engine Braking:
This feature offers smoother, less aggressive engine braking during downhill descents. It may not apply braking if the vehicle speed is not increasing, although it will apply full engine braking force when needed to prevent acceleration. The system automatically activates the engine brake and, if necessary, downshifts the transmission to lower gears to not exceed your vehicle's set speed when the brake or accelerator were last released.

We haul a 5er and 99.9% of the time I use auto with the other .1% I just didn't hit the switch twice to get to Auto.

A @jetjock15 states, these trucks haul so well. Amazingly well. With our 16k# load, we hardly know it's there.