Took that route 7 years ago, and except for an approximately 10 mile stretch at the border of ND and MT, the route was a delightful, mostly 2-lane road. I'm sure that the dirt track for that stretch has been converted to the 4-lane highway it looked like they were building.
We went all the way from Sault St Marie to Redmond, WA, stopping at Glacier, I might add. That is some of the flatest land on the planet, so you should have no trouble at all. It does start getting mountainous in western Montana, but we had no trouble dragging our TT through there.
The one thing challenging about it was the almost continuous headwind we encountered headed west. But it is a great trip, and a wonderful adventure. BTW, we stayed on night in Williston, and with the decline of the oil boom, there might actually be RV sites available there now.