Idaho Hwy 12
I can't answer your question directly, but I'm sure others will jump in. When we were going across Idaho to the Bitterroot Valley in spring of last year I stayed with the interstates for reasons of time, but was told by folks there that Rt 12 was doable.
The reason I'm posting now is this. If you take Rt 12, just as you're about to intersect with US93 in Lolo, south of Missoula, three words: Lolo Creek Steakhouse. If you can't stop, but you're staying around the area for a while, drive back there. You'll be glad you did. :-)
John & Diane, fulltiming since '12
'02 DS 40, FL, 3126, '04 Element
John & Diane, Fulltimers. RVM103 NHSO
On the road since June '12 with Lincoln, the guard cat.
2002 Dutch Star 40, Freightliner, Cat 3126, 2004 Element