We dry camp every year in the winter. The longest has been 7 nights at temps between 25F and 12F.
Tank heater consume allot of battery power. We don't use them
The best tool to have is a muti channel thermometer. We have an older AcuRite that charts the temps over 2-3 days. We place the thermometers by the batteries, pass though, dump valves, outside and an outside wall where the waterline run.
We'll stuff the wet bay full of Roxul insulation. The windows are filled with 1 1/2" styro. The large windows have 12" x 12" and 24" x 24" plugs that slip in and out to allow sunlight in during the day.
The AC ducts and AC intakes are block off with 2" XPS
The top of the slide a 1" EPDM foam that is held on with velcro
Condensation is the enemy. A small inverter and dehumidifier that works at cooler (65F) temps is a must have.
I'm installing a Propex heater. Hoping it will reduce the draw on the batteries during the shoulder seasons. If it works I'll add anther to heat the front of the trailer and the tanks.
https://www.rockymountainwesty.com/Propex_Heatsource_Unit_p/hs2000.htm
A spay bottle and some windshield antifreeze that can be used to thaw ice on the slides.
In a week a 2000w generator will be run (no solar) approx 4-6 hrs a day.
Slide toppers are an added benefit.
A ladder and a bush broom to clean snow off the roof.