Get copies of the Mountain Directory - at least the west edition. They're written for truckers and RVers. Going uphill is just slow, but going downhill can be an adventure. Driving thru the Bighorn Mountains in Wyoming will get your attention, and if you ride the brakes, even in the lowest gear, its possible to boil the brake fluid and then you don't have any brakes until they cool. Routes 14, 14A and 16 all cross the Bighorns on your way to Yellowstone. And all of them have pull-outs with signs that say "Pull out to check your brakes". 16 is the easiest and 14A is the worst. Don't pass by a pull out. The long downhills have runaway truck ramps at the bottom, but then you're stuck uphill and buried up to your axles in sand.
Somebody will soon say that the scenery is beautiful and it is, but when the brake pedal goes to the floor, you won't be noticing the scenery. I caution you to ignore the "Hey, Vern - Watch this" crowd that say its easy. It isn't if you don't have mountain driving experience.
The West and East editions are each $17.95, but the West is the one to buy if you only get one. Best money you'll ever spend. And then you can decide for yourself if it was worth it or not. Frankly, I now avoid the Bighorns completely. I stay on I-90 and turn south on US 191 all the way to West Yellowstone, MT. Nice scenery on that route as well, and I don't have to pry my fingers off the wheel when I get there.