We aren't from Montana, but when we used to have our trailer on a permanent site at the lake, we had water, sewer and power available all year long. We paid for these services in addition to the lot. We would drain the water in late fall, but would still go to the lake and spend a few days there during the winter. We brought water with us and used that to flush with. We did not turn the water back on until it warmed up enough to not have a long, hard freeze. (If the black tank froze for a few hours, it was no big deal. It thaws quickly when it gets warm.)
We would normally turn the water on at the middle/end of April, but we would occasionally have some freezing temps after that, but as long as it didn't stay below freezing for more than 6-8 hours, and then warmed up to above 35, we didn't worry about it and never had a problem. Until we were sure that the freezing was over (usually about May 10), we would disconnect the water hose and drain it before we left, but that was all.
As long as your RV get sun during the day, it will warm it up inside to many degrees above the outside temps.
I hope that answers your question, at least from my point of view.
2017 Ford F350 Crew Cab 6.7L 4x4 DRW