We've taken that route many times usually coming home from Yellowstone working our way back down through Oregon and back to So. Calif in our class A.
95% after you pass Lewiston is meandering road about 45 mph. The only steeper section is going up the Lolo Pass as you enter Montana but it is not bad at all. It is a very pleasant drive and we think well worth it. Toward the Montana end at the start of the pass is a campground called Powell. It has electricity and is on the river. If you time it right and want to camp there you'd hit no traffic going up the pass the next morning.
