Fifth wheel Travel on I-90 from Montana to seattle

West out of Missoula, I would drop down on to US 12. It is a good two lane highway. Yes it has somewhat of a steep decent from Lolo Pass. It is not bad if you are aware of it. Follows river down the canyon to Lewiston/Clarkston
Another of our favorites is WA 821. It takes off near Selah, WA and follows the Yakima River. Some great BLM campgrounds along it.