As said above try to hit the Badlands and the Black Hills in South Dakota. Mt. Rushmore, Crazy Horse, the towns of Custer, Deadwood and Sturgis...the whole area is great.
Yellowstone is great but expect crowds. Glacier is also beautiful also but might be a little off your route.
Montana is a BIG state. The western side is much prettier than the eastern part. If you have time stop in Butte where the largest copper mine is located. Lots of history here.
Coeur d'Alene is the area we are from. While there be sure to take the drive around the lake on Highways 97 and 95, drive up to Silverwood Amusement Park if you love roller coasters. Another great drive is around Hayden Lake. The town of Coeur d'Alene has some great shopping in its downtown area and be sure to go to the resort there.
The resort has a great, yet a little spendy, Sunday buffet that is outstanding. It also has a wonderful upscale restaurant called Beverly's.
Another great place to eat in the CdA area is at Wolf Lodge. the plate size steaks are great.
Also, the CdA Casino south of town has the usual buffet but it's pretty good.
Recommendations for camping in the CdA area are slim. Blackwell is near downtown but expensive. We understand there is a new park north of town but have heard it is one of those that want to try and sell you a lot. Some of those have a get 2 nights free if you'll listen to our sells pitch and hard sell tactics.
You might want to look at Post Falls. It's about 9 miles to the west and there are 2 good parks there. The Coeur d'Alene RV park is the best one and they take reservations. The SunTree is also nice but I don't think they take reservations. If you happen to be an Elks member there is a wonderful RV park at the CdA lodge with Elec and water for $15 to $20 per night.
Have a great time!
2007 Keystone Challenger 5th wheel, Ford F350 Super Duty 6.7L Diesel, Short Bed, 2 dogs and the cat and rolling down the road full time since May 2014