[JRTJH]

Your biggest "heat loss" is going to be the single pane windows and the 1" thick slide walls. Second biggest is the 2" "spun fiberglass" trailer walls. Third biggest is the roof/ceiling with 3-4" of spun fiberglass and a layer of "bubble wrap" (That's your polar package)....

You can use 2" rigid foam sheets on the outside of the slide (sides, top and bottom) to help reduce heat loss. Apply the "shrink wrap window coverings" to all windows you don't need to open. You'll need to heat the space under the RV to protect the "minimally insulated basement" (another part of your wonderful polar package).

Also, plan to have a significant problem with humidity buildup inside the trailer. Showers, cooking, even humans breathing will put a tremendous amount of moisture into the air in your confined space. So, NEVER cook without the exhaust fan on and NEVER shower/bathe without the roof vent open and exhaust fan on. To make either work, you'll need "cross ventilation" which means an open window on the opposite side or opposite end of the trailer. Yes, in -10F weather, it's going to be impossible to stay warm in any "wood and tin box with R-7 walls and single pane windows"....

Essentially, what you're going to try to do is daunting with two adults expecting the worst. Bringing kids into the picture ??? What about schooling and exposure to every other kid in the neighborhood with "sniffles" ??? You'll find that living in such close quarters (essentially a hardwall tent) that you're all going to be sick most of the winter.

It's not impossible, but with 4 people in a small tin box through an Idaho winter???? It's going to be as demanding as any of the "pioneers" faced a century ago. Only difference, you'll be able to get in a car and drive to a motel for the bad days or weeks....