If you have the time, a nice route would be to go north on 120, 72 to Billings and follow I-90 east to Devils tower. On the way, I would suggest stopping at Hardin and look at Little Big Horn National Monument. There are several guided tours starting around 10:00 AM. The hidden treasure is Big Horn County Museum at Hardin. They have about 36 acres with old buildings and many antiques. Great family museum with lots of things to touch. There is a nice KOA at Hardin. We just came east on I-90 pulling a 25 ft FW with a 1/2 ton GMC with no problems. The biggest hill will be on Hwy 14 when you turn off I-90 to go up to Devils Tower and then back to I-90. I-90 into Rapid City is fairly low grades. There is a RV park outside Devils tower. ( no facilities) It is about 400 miles that route.
