Highway 14 & Highway 14A over the Bighorn Mountains in Wyoming 

Time lapse video travelling over the Bighorn Mountains in Wyoming along Highway 14 and 14A from Drayton to Lovell. Consider this option whilst travelling to Yellowstone from the east. Gradients on the west side of the mountains are 10% over a number of miles. I wouldn't consider this for an RV route if you're travelling from east to west, but would be fine in the other direction.
Some interesting sites include the Native American "Medicine Wheel"; a Stonehenge-like site created in the past 400 years. Plenty of lakes, trails and waterfalls on top of the range to explore. We even saw moose in the swamps!
Remember this highway is closed in the winter (this video makes it easy to see why).
Stay tuned for more time lapse videos from the American west.