You have to decide on a control speed for your down hill speed.
Control speed , is the speed at which you feel confident that you could quickly come to a complete stop should the need occour. Depending on the grade that could be 40 to 50 MPH in my coach. I set up at the top of the grade at the control speed and as I go down hill ; watching the tach and speedo; keep the tach between 2000 & 2400 by using the brakes.
When the tach gets above 2500,I use the service brake until down to 2000 then coast again with the exhaust brake , allowing the service brakes to cool. The cooler the brakes the better they will work when you need them.
If you find your having to apply the brakes very often , then you are going too fast and need to slow down to the point where you can gear down the trans and continue in the next lower gear.
The auto gear down feature of the newer systems will not allow engine over reving, trans will shift up when limit is reached. Not the thing you need to have happen , part way down a steep grade.
As I read in another post, you can come down a hill too slowly as often as you like , you can come down too fast only once!