From Helena to Billings you have have the choices right. Hwy 12 is 2 lane almost all of the way and unless it has been upgraded there are areas with small shoulders but very passable in a 40' MH. You will have to be careful in curves to be sure you stay on your side of the road. This is common problem in MT so if you take 12 be careful. the alternative of I-90 is interesting in that you should leave Helena on US 12 to Townsend and then continue to Three Forks and I -90 on MT 287 which is also 2 lanes. The last time we were on it there was construction improving the road significantly. Since that was years ago, it should now be very good. I-90 is good and from Three Forks there is only one mountain pass, just east of Bozeman and one long grade just west of Columbus.
If you go through Three Forks, there is a great bakery and deli just before you get to I-90. Great baked goods, soups and sandwiches. We always stopped there either to eat or pick up food for later. In town there is the best root beer stand, Custer's Last Root Beer Stand, that is worth a stop but there was no parking for the MH, only cars. :-(