Keep in mind that when a mapping program (or gps) recommends a route, it has calculated the distance (and time estimate) on many possible routes, and presented the shortest (or fastest). It usually choose freeways simply because the speed limit is higher.
By 212, I assume you mean the piece in SE Montana. As the other poster showed, it's been discussed before. It may save a hour over staying with I90, but it is more remote. Your program recommended it solely because of that time or distance saving, not because it is better. For you peace of mind, I'd suggest staying with the freeway. You'll have enough 2 lane highway driving later.
What was the US2 recommendation? To get to St Mary you have to first get to Browning (and do take MT464). To get to Browning you have to use either US2 from the east, or US89 from the SE.
US2 is the main highway across the top of the country. Some RVers drive it all the way from Michigan. It is also a good way of crossing the Rockies to the west side of Glacier.
But if you are coming from the I90, a route including US89 is more direct. The drive from the Billings area to Great Falls is mostly flat open country, with the exception of the Little Belt Mtns that US89 crosses.
Even if you map does not show mountains (but it should), it will show National Forests. In this part of the country, NF are located in the mountains, where trees grow well, and ranching and farming is harder. These are also the more scenic areas, but a bit more challenging to flat landers in big RVs.