Mountain Driving

I believe the gasser doesn't have the Tow Haul mode where you can tap the breaks and slow it down like diesels. When we had a V10 truck and towed a travel trailer, we had to do the slow lane with the semi's and slow it way down, then pump the brakes at the end to keep them from seizing up.

When going down hill I use the tow haul mode weather towing the toad or not. if you hard tap brakes tranny will down shift if that is not enough hard tap again until you get to speed desired. Tranny will hold in that gear until you give gas and then will shift up again. On long grades I slow down at top so I can control the speed starting there don't wait until you going down to fast then start trying to slow down. If you need to use brakes I apply hard to get down to speed desired and let the tranny work after that don't ride the brakes. It's easy once you get use to your coach and controlling the speed.

Enblethen hit key points. Having lived decades at 9000' I'd add on ascents be happy with slower speeds. Those 18 wheelers in the right lane going 5 or 6 miles an hour, you may have to do that, depending on how your gasser behaves at altitude. You will lose approximately 3% of your horsepower for every 1000 ft above sea level. And I would reinforce the idea of using your transmission to hold your speed on the downgrade. Keep your foot off the brake. When you get up a few miles an hour faster than you like, that's the time to use the brake just to bring it down four or five miles an hour. That way they will be available should you need them in an emergency. Nothing worse than having hot brakes fail when you need them the most. An aside, those emergency off ramps will just about tear your vehicle apart if you have to use one. Truly last ditch option.

Don't try to do the posted speed.
Let the transmission shift down on its own, then when it starts searching for a gear select it manually.
Shift transmission to lower gear descending steep grades.

Use low gears. Generally the same one you used to ascend. Never ride your brakes. Slow down using your brakes and transmission to 15 to 20 mph below what you consider safe. Don’t use any brakes until you reach a speed just over what you consider safe. Pull off and let the brakes cool anytime you believe necessary. It is not a race. Learn to enjoy descending. Ignore those behind you until you can safely pull over.