Yarnell isnt particularly sporty to drive. If going up take breaks if your motor gets hot. If going down, remember to gear down, yes, even with an automatic transmission. Thats why they have lower gears you can manually shift into. Its semi-common to hear of some hapless motor home users who bad a brake failure on some steep long mountain road (far larger hills than Yarnell) and crashed. If they geared down they would likely not have cooked the brakes, and would have had to have a transmission failure first to end up with a brake failure. If the trans goes out on a downhill run while engine braking, its time to use the brakes to STOP, not continue down the hill.
Coming back from a sawmill in the mts with a 6x16 flatbed trailer stacked deep with lumber and firewood, I gear down and drive about 20 mph for about 5 or 6 miles on the main downgrade with my suburban. One guy that was with me once asked, apparently somewhat perturbed, if i was going to drive that slow all the way down, to which I replied yes. He didnt have anything to say after that/ No idea why its important to drive as fast as you can barely keep a vehicle on the road and cooking the brakes all the way. Ive smelled seriously cooked brakes on many vehicles that just came off a long downhill grade in the mountains.