Bubble wrap on the outside of windows.
If the fridge is in use it may need to be winterized. (See the winter camping thread under the full time forum).
Skirt the trailer.
Consider using a porta potty for waste.
crack open a ceiling vent, and open the window farthest from it. Place an electric heater near the window.
Find a low profile oil filled heater for inside the skirt.
Run an additional power cord to the RV (I have 3; OEM 30, a 20, and a 15 amp).
Be prepared for HIGH power bills.
Monitor the temperatures inside the skirt.
Regards, Don
My ride is a 28 foot Class C, 256 watts solar, 556 amp-hours of Telcom jars, 3000 watt Magnum hybrid inverter, Sola Basic Autoformer, Microair Easy Start.