Some info on our insulation and heating.
Temperature conditions:
Cold is my enemy. I would say -10 to -15 C would be common in winter times. -22 would be rare. -30 C would be once in seven years and -35 would be ones in twentyfive years.
This is about the same as we experience where most places we travel here in Norway, and was a big part of our own considerations.
I would like to have reasonably comfortable temperature in my housing unit say 11 to 14 C (? what would be realistic with reasonable heating in -15 C?)
We have a Bimobil EX480 (see Bimobil.de) with standard walls, floor, roof thickness. Heating is from a Truma 6E gas heater. When it's about -15 we use one 11kg gas cylinder pr. 3-5 days depending on inside temp, and much we use the hot water, open windows/doors and cook. With an inside tempearture in the 19-21 degrees range, we last slightly under 4 days on average pr. bottle. This is a lot better than what we have experienced with other campers and what many report as normal use, mainly due to the good insulation of the Bimobil camper.
Truma reports 480 gr. as average hourly gas consumption on maximum (normal for winter operation), that should give 11,52 KG use pr. 24 hours (about 1 cylinder pr. day). This means the average 4 days gives me a 25-35 % gas usage compared to the "worst case" scenario. A good number!
The Bimobil floor is 60mm, walls and roof is 45mm. Check their site for specs on the foam they use. We also have some cold bridges in the camper, so it could be even better with careful planning of the construction.
Windows and such will be the biggest cold spots. I plan to build option to replace such things with equipment designed for winter conditions (insulation top priority)
We have not done anything special about window insulation (and use the standard dometic RV windows.) I have thought of making a system with four pushbuttons around each window with a "curtain" of insulation using the typical "front window" insulation panels one can find (wool inside, reflective material outside, cross stitched.)