While I would recommend MT 464 for long trailers & large motorhomes, as mentioned in an earlier post, another alternative to US 89 all the way from Browning to St Mary is to take Starr Schoolhouse road out of Browning. Flat & wide, it skips part of US 89, although you will still hit some hills & curves once you are back on US 89. Now I have to admit I'll pulling a 17' trailer!
I'll second skipping MT 49 - even with my small trailer it was something I'd rather not do again. Very short switchbacks & many frost heaves.
