[Daryles]

Go around under your rig and close up all the holes and gaps. This will help keep mice and drafts out.

Skirting (put some heat ,light bulbs, milk heater, under there to keep it warm), also cover the slide roof, walls and floor with 2" styrofoam insulation.
In the picture below, I would skirt under the overhang and put a door there. Tool shed, snow shovel etc.

For the windows, reflectex on the inside (3 layers).
Some use 10 gauge clear vinyl over the outside of the windows (lets light in) using air conditioner foil duct tape (removable without a residue, make sure the surface is clean) and another layer over the inside of the windows. This creates 2 dead air spaces. Prevents condensation on the windows.

The key is getting it done before it gets cold. It's difficult to get tape to stick when it's freezing.

You will need a way to dump your tanks.

There is a Forum section, "Burrr, it's cold outside, winter camping", for more ideas.