US 212 from Red Lodge into Yellowstone is one federal number road that most RV owners shouldn't take. Not a bad road, but just a lot of high altitude climbing. Teton Pass from Jackson to Idaho (22/32) is another pass you should avoid.
McKinzie HWy OR 242 is off limits to longer rigs because the summit area winds through a lava field with sharp turns. I haven't driven it myself because it hasn't been open when I've passed the area (late spring snow).
In the Columbia Gorge, I84 is freeway, allowing the driver some chance to rubberneck. US30 lets you get closer to the forest and waterfalls, but is an old, pre-RV, route. The mountains on the Washington side aren't as high, but WA14 lets you look at the Oregon side from across the river - just be sure to do that when stopped!.
You can drive on paved forest service roads from Carson to Randle, with a side trip to the east side of Mt St Helens. But you do need to be comfortable with grades and curves.
Learn to use online mapping tools like Google Maps. With terrain mode and Streetview, it is possible to 'drive' most paved routes (and some gravel) in the western USA. Practice on familiar roads to get a feel for how those map images reflect the real world. We have far more information roads across the USA than we did decades ago with paper maps and AAA routing guides.