Another "away from the freeways" route, at least in the summer is US2. I've driven across the US on all the interstates & most of the older US routes, and 2 is my favorite. Mixed sections of 2 & 4 lane roads, interesting small towns along the way.
If you arrive at Glacier National Park after the Going to the Sun Road is open (usually late June or early July) be sure to drive through the park. You can't take your Class A, but if you don't have a towed vehicle, there are shuttles or better yet (but more expensive) one of the red Jammers for the drive through the park.