A good rule of thumb is to go down the hill slower than you climbed it in 1 or 2 gears lower than you used going up.
I was a trainer for long haul truck drivers and here is my mandatory advice to every trainee.....
IF YOU HAVE TO USE YOUR BRAKES TO SLOW THE RIG YOUR GOING TO FAST
reason= if you use up your brakes your either going to have to use the truck escape ramp (destroying your rig) or your going to be dead
YOU WILL ONLY GO DOWN A HILL TO FAST ONE TIME (BECAUSE YOU WILL NOT BE ALIVE TO TRY IT AGAIN)
Now on big rigs you have a Jake brake ... my reasoning is that if you go down without using your brakes if something fails like the JAKE (engine brake) or the engine dies then you still have all your brakes to stop you..... however if you heat them half way to failure point you won't know it and if there is a failure YOUR DEAD OR YOU WILL BE CLEANING YOUR DRAWERS AT THE LEAST
While your learning your new rig stop at the top and bottom of every grade and walk around your rig checking for brake smell and any signs of heating including feeling your tires and rims......feel your rims carefully because they might burn your hand..... motorhomes and travel trailers should for safety follow the speed limits placed on trucks when doing downgrades .... I personally suggest going slower than the recommended truck speeds... at least until you know your rig And never never never exceed those limits..... Always stop and check your brakes at truck brake check areas
To aid in slowing your rig turn on all engine loads you can such as the air conditioning even in winter..... On a big truck turning on the a/c as well as the engine fan (they usually have a manual electric fan clutch) will save your brakes and hold you back a noticeable amount. Just like turning off your a/c when climbing a Hill Will allow more climbing power turning it on going down will pull the engine rpm Down
Following the things I have advised will keep you alive and will save you money and time on brake jobs
Like I said above you only go down a hill to fast ONE TIME
KEEP YOUR SELF AND YOUR FAMILY SAFE AS WELL AS ALL THE OTHER DRIVERS YOU MIGHT RUN OVER IF IT GETS AWAY FROM YOU
if your rig has air brakes and your using your brakes going down you could suffer brake fade and your dead or you could use up your braking air and your dead..... hills are not to be taken lightly
While your learning your rig pay close attention to the signs telling you the grade percentage and the length of that grade.... If it don't tell you how long the grade is assume its the longest......
And my last bit of advice .... Once you learn your rig don't get brave because engines are known to fail on the down hill from the work they just did to climb that same hill..... with air brakes you only havea limited amount of air to stop you.... On rigs with conventional brakes if the engine dies you lose your power assist ... either way you will be fighting to stop .... If the engine fails (and even new engines die sometimes) put on your emergency flashers and stop the rig immediately As fast as you can safely stop getting as far right as possible .... even if the road doesn't have a wide shoulder
..... HILLS DO KILL