Well we were in Northern MT a few days ago and decided that there is such a lack of spots to camp east of Shelby MT on Hwy 2 that we dropped down to I90. The very few places on Hwy 2 have ratings of 2 or 3 on Campgroundreviews. Yes there is the Ft Peck cg and the reservoir for boondocking but the cg was booked solid (with a few walkups) in the middle of the week which left us concerned since we'd be getting there after 4 PM and not finding a spot would be a problem. Last time thru we had to motel it in Glasgow.
Middle MT has plenty of places to see and camp at off I-90