We drove the reverse of you last spring.
I90 was a breeze, stayed in Mitchell, SD
We camped at Custer State Park-Game Lodge and used this as a base from Badlands to all the Caves, and monuments.
We drove out to Devils Tower NP and Deadwood from there. We were there a week, and were busy everyday. I made the reservations a good bit ahead, it wasn't when we were there, but I understand it can be crowded and forget about it during Sturgis, they're sold out then. (try to do the wildlife loop and the park rangers were excellent there).
From Custer SD, 84n to I94 (tough crosswind) to Medora. (TRNP)
Red Tail Campground (tight)
I94 to Fargo (wallys).
I94 to Chippawa Falls Wi (Leinenkrugel Brewery-free samples!). Very nice Wallys there.
The interstates ended there for us and we started cross country. None of the roads were difficult
Take the sunset boat tour at Picture Rocks, but ALWAYS dress warm. We went out July 2. Went prepared for the night cool, but was still cold.
The Soo was a good stop.
If you're going through Duluth, I hope you stop at the Apostle Islands. We were disappointed we couldn't make it there.