You can start with the 7-day itinerary to get a general idea of possible stops:https://www.trippy.com/drive/Chicago-to-Lethbridge-in-7-days
Most of those days are 4 hours or less of driving. The only exception is between Glendive and Shelby, Montana. For that segment of the trip, click on "find more stops" to zoom in. Then you can change that segment to 3 days:https://www.trippy.com/drive/Glendive-to-Shelby-MT-in-3-days
So it looks like one option would be:
- Chicago to Madison
- West Fargo
- Wolf Point
- Shelby (Montana)
That's still under 10-12 days, so another option is to change your route. I would personally go through South Dakota and spend time in the Badlands National Park, Black Hills National Forest, Custer State Park, and of course Mount Rushmore if you haven't done it recently.
Or alternatively, go from Chicago to Winnipeg and spend more time in Canada. Either way, having more days gives you the luxury of exploring each stop at a relaxed pace. Have a great trip!