OK, so we thought we would take a trip to Montana in Mid August with a 35 ft gas MH from Massachusetts. Some have said it is too hilly and some would really be very scary on the brakes and high grade...
What we recommend depends on the time you plan to travel. I recommend at least a month. Going from/to Yellowstone and Rapid City would recommend you stay on I-90 to Bozeman then 191 to West Yellowstone.