A hard question to answer. People live in RVs in the northern Rockies, but it takes some experience to be comfortable. You have to learn how to keep your plumbing working and stay warm.
My point of view is that most RV people can be pretty functional down to around 0-10 above by doing the obvious. Especially if day time temps make it above freezing.
Spending the night at a ski area parking lot in the Rockies with no hook ups is not so easy. It can be -35 degrees F or more. I have slept in the back of my insulated pickup with no heat at around -20 degrees with few problems.