On that route I would think the scariest driving would be in the big cities (LA, San Fran, Portland, Seattle etc) as opposed to anything you might find in the mountains or on the Highways. In the mountains you can just slow down and take your time. In the cities, slowing down often encourages people to do crazy things to get by or around you (or to let you know they think you're driving too slow). The I-5 between Portland and Seattle is especially hectic.
The highways between the Canadian Border and the Banff/Jasper area are all very good and well travelled. Yes you will find some grades but there are lot's of passing lanes, pull outs and rest area's as well. The Icefield's parkway between Banff and Jasper is easily one of the most beautiful drives in North America.
We visited the Yellowstone/Mt. Rushmore/Crazy Horse areas last summer and found all the roads to RV friendly.
Hope you have a great trip !!
