Highway 12 from Lewiston to Lolo is fantastic. To be clear, I did it on my motorcycle so take this with a grain of salt, my recollection is, it's a long climb going west to east but not a particularly technical pass. Should have an easier time of it than Lewis and Clark did, in any event. Wound up staying in Butte which is a fun town.
It's *really* hard to find a place to camp anywhere near Yellowstone right this minute. If you want to do the west gate, 191 out of Bozeman is really pretty, haven't done it in the rig but it runs down the valley so shouldn't be bad. If you don't mind dry camping, call Valley View, they should be able to hook you up for cheap. That's what I'm doing, in fact we may be there at the same time. Don't count on finding national forest camp sites anywhere near there. I'd spring the 35 bucks to stay at Valley View and hit the west gate before 6am, blow through the gate for free, avoid the traffic, get a parking place at old faithful and do the tourist thing...
Are you gonna loop through Colorado out of Casper? Gorgeous country.
60 is a nice ride through MO, drops you off in Paducah
2008 Tiffin Allegro Bus