Billings MT to Coeur d'Alene ID

I have never encountered an 8% grade on an Interstate. 6% is typically the max. If you have crossed the Rockies frequently then you know that its more about the length of the grade. Some can go on and on and that is usually the worst of it. I didn't encounter anything bad on the route you are taking. The very first KOA in the country is in Billings. There is a nice RV park on the south side of Missoula, and another nice park in Couer d'Alene. Mostly just a very scenic drive and nothing worse than Loveland Pass which you have probably done before. And easier than that long grade between Salt Lake City and Park City, for example.