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.

Certainly consider the route that enblethen suggested, but I-90 is also a pretty drive......though, it is still an interstate! The Lolo route would be a great choice.....maybe I-90 for the return trip.

You may consider WA 20 for return trip east out of Mt Vernon, WA
Or US 2 east out of Everett, WA.
Both are good two lane highways.
Could take US 12 all the ay west to I-5

All of the routes are quite scenic but stay out of Seattle unless you need to be there. US 2 is an easy drive although traffic gets heavy as it approaches I-5. 12 is not a difficult trip either.

Pulling a 40 ft. fifth wheel on hwy 12 is doable, however as I recall there is a sigh telling of 70+ miles of curves. Depends on how you like twisty roads. Interstate 90 is a very nice trip and many picturesque towns such as Kellog Idaho and Coeur d Alene Id. By the end of Sept. some Federal Campgrounds close. Do your research. We have traveled Hwy 200 also and it is a very nice trip.

We do US 2 west from Spokane to Wilber then north to grand Cooley and work our way to Brewster and the north cascade high way to Burlington Wa and I-5

I’m sure I do t know how to use this yet but for all those who replied THANK YOU.
Now I need to know about campgrounds on same route. Are there plenty or at least a few?

My husband wants me to ask about any low tunnels which I’m sure there are non as you wouldn’t have sent us this way. I just want to get off interstate. That’s how we travel mostly and I want to see homes and stuff. Just not interstate stuff

Us 12 has many US forest service campgrounds. Only a couple have hookups.
WA 821 has BLM campgrounds with no hook ups.
No clearance issues on US 2, WA 20 WA 821 or US 12.
Commercial campgrounds are available along most of the routes.

Get the RVParky and ParkAdvisor apps if you have a smart phone. You won't find too many forest service campgrounds anywhere in the west that will accomodate a 40 foot 5th wheel (I have a 35 foot 5th wheel and travel a lot in the Pacific Northwest). The only one I know of on Hyw 12 is the Powell Campground just west of the summit of Lolo Pass. It has maybe a half a dozen sites that would take a 40 footer (I stayed there about 3 weeks ago). Private campgropunds are another matter.

https://wenatcheeriverpark.org/ and https://parks.state.wa.us/541/Lincoln-Rock are two of our favorite spots near Wenatchee on Hwy 2. The fall colors on Stevens Pass on Hwy 2 to Western Washington are great in Sept. Before reaching to top of the pass there is a wide shoulder to pull off on, and get out and look across to the hill side colors.