I am living in a refurbished camper right now. Do not attempt this in a regular RV or Camper. They are not designed for -40 temperatures. My husband and I ripped it apart three years ago and added 2 inches of spray foam and 2-inches of cellulose to make it as winter-tight as possible. The R-value is something crazy I can't think of right now.
Most RVs and Campers do not have the R-value for extreme temperatures. We built our solar-panel system before it was cool or came with tax breaks, and we are currently upgrading from 24 to 48 volts once our battery is delivered this week.
I will say do not attempt this unless you are willing to live hard. Currently, all our water is frozen, and our tank split as we tried to move it, so yeah, we have no water for the winter.