I am living in a van here in NZ pretty much year around.
My van is Toyota Hiace which I have been building for 3 years while living in it.
I have used wool insulation on the walls and 10mm polystyrene under the floor panel. I would have put thicker insulation on the floor but it cuts into my headroom which is just enough for me to stand in.
On the roof I have 25mm polystyrene blocks under the felt.
Cooking causes a lot of condensation and before winter I should put in extractor fan over the stove.
One important trick is to maximize the heating effect of the Sun and park such a way that you get sun heating the van in the morning.
Also I try to chop my travel so that after cooking dinner I travel to another spot to spend a night. that way I can use the van heater to heat the van before I go to sleep.
I have thought about inventing a micro wood heater which would use small sticks, pine cones or saw dust? Feel free to steal this idea as I would prefer to buy one off the shelf rather than spending money on developing one.