RECOMMENDED ROUTE FROM PDX
I've made the trip from Portland to Montana more times than I can count, this I have found to be the best route in scenic value and twisties while also maintaining the most direct route (without having to endure freeways for too long.) I've pin pointed natural gas stops, a few rest areas, restaurants in the Bitterroot Valley, along with my recommended camping sites.
I normally do the trip in two days, stopping at Pierce's Green Valley RV Park in Touchet, WA on the way to MT, and Clearwater Crossing RV Park in Orofino, ID on the way back to PDX. The 500 or so miles is doable in one day. However, it is a bit physically exhausting. From fighting the winds down the gorge and through the Umatilla area, then climbing Lolo Pass, it's nice to have a break in the middle.
In addition there's a bunch of lovely stops marked on the map, to take a minute to relax and stretch your legs. Enjoy your ride!