I've been down some steep hills with my 2000, V10, 30 ft class C, pulling a toad.
With your foot off the gas, gas powered engines, are like natural engine brakes.
With the normal hills I press the O/D OFF button on the end of the shifter. When the light is on the transmission drops down to 3rd gear. The engine speed goes up but that helps slow you down.
If I feel the speed, still picking up on steep hills, I move the shift handle to "2". In that gear you can go between 45 and 50 MPH. If it's that steep, you don't want go any faster, anyway.
A saying is, always go down a hill in the same gear you go up it in.
I on a few occasions we found our rig on unpaved, real steep, mountain roads. In that situation I put the shifter in "1" to keep the speed down around 20 MPH. If it got to fast, I would use the brakes to get slow again and then let off and let the engine do it's braking thing.
Get your rig on the road and up to 50 to 60 mph. Take your foot off the gas and hit the little shifter button, you should notice it slowing down quicker.
Then move the handle to "2". You will see how quickly it slows.
Practice, practice, practice.
For other readers of this post, we do not have Tow/Haul 5 speed transmissions.