Have you ever taken a winter nap in your car? If you don't keep the heating on, it takes two minutes and you start to feel the cold outside.
It 's the same thing in a camper , if you don't run for cover, the outside temperature will be the same in the driver's cab. It is absolutely the point from which you will lose the most heat. First, set the air intake selector to recirculation and close the vents , at least no gusts of freezing air will enter. The doors, at the bottom, may also have air vents, close those too if possible.
And then… let's take care of insulating the cabin.
The basic concept is to stop the cold as soon as possible , preferably outside the cabin. So the best thing would be to use an external full-face thermal blind . Excellent for the classic white week, when temperatures of -10 are the norm. Better if made up of two pieces, because it is more versatile in use and convenient for stowage. It must be purchased specifically for the cabin model on which to apply it. For snow habitués, especially for multi-day stops, I consider it necessary .
Alternatively, you can do your winter outings anyway, shielding the cabin from the inside , even if the efficiency will certainly be lower.
Anyone who has an overcab with a classic dinette can quite easily separate, even on the fly and in an emergency, with a blanket (fleece is perfect) brought down from the overcab to the ground , and to secure it, slip it between the mattress and the floor same as the attic. If there is, draw the curtain around the cabin, even if it is in summer, and do not put thermal blinds on the windows , otherwise ice will form between the blind and the glass; this does not apply to temperatures above zero , a situation in which thermal blinds will be very useful. In the morning the air in the cabin will be freezing (like everything you have left there) but the blanket will have done its job shielding the cell. For a "more professional performance", with a little dexterity you can create a partition with multilayer fabric, cutting it to size and applying it to the overcab profile (also on the sides) with snaps, the best ones can also create a passage (with zip or other system) between cabin and cell. For those wishing to buy it, there are excellent ready-made ones . However, I am not a supporter of cabin insulation, also because it is useful for storing some somewhat bulky things during the night. I prefer to have a good separation around the perimeter of the cabin, as in the case of the semi-dinette.
For those who instead have a semi-dinette , the best "internal solution" is to put a thermal curtain inside the cabin , to be clear along its entire perimeter, making it reach the ground well, preferably a few more centimetres. Generally there is already a curtain that goes around the cabin, but it is not heavy enough, it does not reach the ground, it is often not wide enough, however we can use its rail to hook the "winter" one. Unfortunately this solution is not feasible "on the fly", you will have to prepare the tent, just as you will have to do a test to hook it to the cabin. Choose a light but thermal fabric, a fleece will be fine, sew a curtain tape on the upper edge, so that you can place the classic hooks on it. A tent like this can be easily stored in any locker, it is small and light. Obviously it can also be made by those with the classic dinette.
Placing thick cotton mats on the floor, such as those used in the kitchen, will allow you to create some insulation from the floor to reduce the cold on your feet, and to absorb any water and dirt you bring inside. Personally I prefer the mat solution to that of a fitted carpet , even if the latter would insulate much better, because it allows them to be replaced frequently. In front of the entrance , internally, it is convenient to keep an absorbent non-slip mat on which to stop to change shoes before entering.
Personal opinion
I don't like the divider between the cabin and dinette , I tend to prefer having a larger space available and being able to use the driver's and passenger seats (even if they aren't swiveling), always useful for keeping the living area tidier by placing bulky things on it. Furthermore, by separating it from the cell, the cabin becomes really cold.