Judy, we have done that exact route 5 times. We found it very beautiful in all states except the eastern half of Montana and a good portion of North Dakota, not that it's not pretty, but it's mostly flat.
It's a great route though and the only one we take out there anymore. Elevations are not as much of a challenge. Diesel fuel is readily available. Traffic is fabulous, especially compared to any interstate. I even have notes somewhere in regard to stations and campgrounds on the way. Minnesota, Wisconsin, the UP in Michigan and the Lower Peninsula in Michigan are very beautiful. The ride through Montana on the Southern end of Glacier is fantastic.
If you've not seen Glacier set aside a few days and do so. Lake Flathead is gorgeous too.
It is an exciting trip and I am envying you big time. *Sigh
We can't go this year and I'm bummin so. We have property in the very North West corner of Montana and love the state. We plan to retire there someday, hopefully soon.
Which way will you be coming to get to Flathead?
Take care,
Patty