Down hill grade

Find a gear and road speed that KEEPS YOUR SPEED IN EQUILIBRIUM-- you are neither speeding up nor slowing down.

Doesn't matter it the road is dead straight, if a steep grade, that speed may be 15 MPH and 1st gear.

If you have to use the brakes (brake pedal) do so firmly and long enough to drop to the next lower gear.

And, if hydraulic brakes, it is CRITICAL that your brake fluid is fresh. Fresh brake fluid boils at over 450 degrees F. Hydrated/old brake fluid boils below 260 degrees F. Boiling brake fluid= substantially reduced braking. Easy to check. Open the master cylinder and check the color. New brake fluid is absolutely clear-- like water. It darkens as it absorbs moisture (which it does to keep the ferrous metal brake components from rusting). Light tan may be OK in the short term-- darker IS NOT.

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.

Assuming it's a gas engine. (diesels are a different game)

Generally start by putting it in the same gear you rode up the hill in (assuming the downside is a similar slope). Yes, the engine will rev but unless it's redlining the engine, shouldn't hurt anything and the reving will provide some engine braking.

If it does start to go a bit too fast for comfort, use short but firm braking followed by releasing the brake entirely. That will allow the brakes to cool down between applications. If you just apply light braking, the brakes will overheat and then when you do need strong braking, they may not work.

Obviously, watch traffic and curves so you can adapt.