When I drove semi's or towing my own heavy trailer rule of thumb is travel down the hill at the speed you would be traveling if you were going up the same hill.
braking. Apply braking to reduce speed to mph you desire then let off. Going 65 and need to get to 55, well brake until you get to 55 then let off. NEVER ride the brakes they will overheat and fail. That's why there are runaway truck lanes.
Using tow mode and engine revs high, they are designed and can rev high, if revving too high use brakes to reduce your speed as mentioned above.
It's always a good idea to just reduce your speed going downhill. Use common sense, you will be fine.