Let there be no conflict in your information. The answer is NO. Do not attempt the length of that road from St. Mary's to West Glacier. I have been up and back down it from West Glacier to the pass at the peak in a Jeep Liberty, a smallish SUV, and would not want to have taken any larger vehicle on that journey. I do not know about the other side, but given the nature of the road I was on and the traffic on that road, you will not like the journey in a 25 foot RV, even if they let you make the attempt. Go around and take the bus. Trust us on this one.