Missoula to West Yellowstone

Take I90 south to Belgrade, MT. There you turn due south on MT85 for a few miles till you come to the intersection with US191 headed due south. Keep going straight on US191 all the way to West Yellowstone,MT and if it were me Id stay at the Grizzly RV park there

I might add when you get to the Jct that says West Yellowstone right or Quake Lake left you can take the route through Quake Lake where the earthquake caused the huge landslide that buried the campground and some campers and formed the lake. Several pullouts if you want to take some photos. I generally see some mountain sheep along this route. Both routes go into West Yellowstone.

I go this way every year. Stay on I90 to the Cardwell Jct just past Whitehall, MT. Exit at Cardwell onto Hwy 359. Stay on 359 to the Jct with Hwy 287 & take 287. Goes past Harrison Lake & through Ennis, MT a unique little burg. Stay on 287 south out of Ennis to the jct with Hwy 87 and stay left on 287 to jct 191/287. Stay right on 287 to West Yellowstone. I take this route every year in my coach & toad to visit the park.