I realize I'm reading between the lines, and may be misreading...but if you're thinking you will save money overall in Montana by full timing in a class C RV vs. renting an apartment or something similar, particularly over the winter, I suspect you'll find you're mistaken. Check the seasonal rates at campgrounds, and figure you'll spend a good bit on propane (or electricity) for heating as an RV is nowhere near as well insulated as a house.
If you're thinking you won't be in a campground, figure out where you'll get water fills and sewage dumps and propane fills when you need them, and preferably have an electric hookup. (Solar is nice, but you'll need a comparatively substantial system to keep enough power to run the furnace when the days are short in winter).
If you aren't looking to try to live on the cheap, but have other reasons for wanting or needing to full-time in the RV, then maybe this doesn't apply quite so much. Likewise, it's a little easier if this is a seasonal thing during the warmer weather.
