Re: Suggestions WANTED-Montana & Wyoming!
If it were me, I would leave Billings and drive to Garryowne, on I-90 to the south of Billings the first afternoon. The next morning take time to visit Little Bighorn Battlefield Natl. Monument. From there I would continue south to Ranchester, WY where I would go west on US14 over the Bighorn Mountains to Greybull and then west to Cody. I suggest at least one day in Cody and a visit to the Wild Bill Cody Museum. There are RV parks nearby.
From Cody, continue west on US-14 to Yellowstone, where I would plan to spend at least three days. As suggested, you will need reservations. You may want to spend one night at the campground in Grand Teton NP that is near Jenny Lake.
After Yellowstone, you may want to leave via the west entrance at West Yellowstone, MT and then follow US-287 north to I-90 and then to Butte which is an interesting old, historic mining town. Continue west on I-90 to Missoula, which you may want to spend a day in, if time allows. From there head north on US-93 to make a stop at Flathead Lake, and then on to Kalispel where you change to US-2 to Columbia Falls and on to Glacier NP. If you are up to it, you can follow US-2 when you tire of Glacier to the east side of the park and then make a side trip up into Canada to visit Waterton Lakes NP there. As mentioned, you will need passports to return to the US after the visit, but it is well worth the time.
From there I would head south on US-89 to Great Falls, and while you could take I-15 south to
Butte, I would stay with US-89 back to I-90 which will then lead east for your return to Billings for departure to your home.