I did that trip a few years ago from the Detroit Lakes Mn area. I was meeting my wife in Glacier, she and a friend had taken the train and I was to meet her in Glacier. Since I was traveling alone I took 3 days to make the trip. I spent the first night at Fort Lincoln state park near Mandan ND. The next night was at the campground at Fort Peck MT. The next night I spent at a FS campsite at Maria Pass on US 2, then into a NP campground on the East side of the park for a few days before returning to the Maria Pass campground for a, few more days.
We made the return trip to MN via FS campgrounds in the Little Belt mountains, with a stop in Medora ND.
If you get a chance take the Red Bus tour.
