We traveled rt. 191 south to West Yellowstone a few weeks ago for the first time. Decent two lane road. If I recall, most surfaces were in good shape. A few grades but I don't recall anything greater than 4% to 6% and nothing extremely long. After 5,000 miles, the memory of every highway is fading
Perhaps some of the more local folks can add more detail. I'm told weather can be a concern that late in the season, but I'm sure you've got that figured already.
PS - close call.... had two Bighorn Sheep run just in front of the MH a few miles before West Yellowstone! That was really cool, but too close for comfort.