Many posters recommend MT464 (Duck Lake Rd) over the part of US89 south of St Marys. 464 runs straighter and flatter, while 89 winds through the foothills.
My personal experience in a car was that this part of 89 was ok, except that I drove it at the end of a full day's drive, and it was tiring.
I have not driven MT49, but on Google Maps (terrain and streetview) if appears to be as windy as 89. It looks scenic, and would be fun in a car, but might not be in a large RV.
So 464 and US2 is the straightest, flattest way to get to the south and west side of the park. Keep in mind, though, that the park is on both sides of the divide, even though the main facilities are on the west.
AB/BC 3 is another way to the west side of Glacier.