Enthusiast I
Go to the nearest Lowes or home depot and purchase a role of "Reflectics" it is 48in tall by about 10ft long and is a silver Mylar insulating foil material similar to a car window heat shade, but much, much better, cut the material to fit your windows and you will see a dramatic difference in heat and cold protection. It can also be used around the perimeter of a tent or under the sleeping bag to reflect heat.This is awesome I'm living out of my 4runner right now and I've got all those bases covered except for window fog overnight. Anyone know how to prevent that?
Here's a good video by @vagabondexpedition about bedding and window nets to keep condensation down and bugs (not a big worry in the winter, but overall.) Even in the winter cold, leaving a window cracked will help with condensation, but not totally eliminate it. Once in 9 degree weather, sleeping in my Jeep, the condensation was so bad, it actually made stalactites on my roof/hardtop! HAHAHAHA!