hey if your coming in from Missoula, you shoul let me tell you the most beautiful drive I have ever done.
Driving from Chicago, I went through missoula and south down onto 12 at which point I turned immediately around and slept for the night in a hot springs(pool) parking lot, due to the INCREDIBLE ammount of wildlife on that dark and empty road. Good thing to, because I was able to view route 12 in all its glory through walla walla oregon & walla walla washington, connecting to the columbia river gorge etc.............................Beautifull.....Hmnnnnnnnnnn
Anyway you should probably camp in the high desert anyway.
keep an eye out in the walla walla's for the strangest "road signs" ever (written on the road in paint)