We get this question a lot: Is it possible to sleep in the car in winter times? Yes, it is.
Most Arctic Campers (exept the Basic Camper) do have a Diesel parking heater. Imagine this heater like a little stove, built under the car. It runs on Diesel or Gas (Bensin), which it gets from the regular Tank and it takes very little. Maybe 0,5 liter per hour on full power. No need to keep the cars engine running for the heater. The heater also has a ventilation system, that distributes the warm air in the whole sleeping compartment. So therefore the heater needs electricity. That´s why we also put a second (or even 3rd) battery in the car. This battery is even larger that the one for the cars engine and it will be charged while driving. If you shut down the engine, the extra battery will be disconnected automaticly to ensure that you will be able to start the engine again the next morning - no matter how much you will use the heater. Bonus: This battery also powers the fridge, lights and USB.
Let´s say you will be driving 1-2 hours per day, the heater will work your whole trip. In fact, after 3 years of Arctic Campers, we only had one guest ever who managed to run out of power for the heater. Turned out they found a very nice place and they stayed there for three days straight without moving the car and keeping the fridge running, too.
About the temperatures: We don´t know about your personal comfort zone and where you are going to or how cold it´s going to be. We had guest in winter times who were complaining, that the heater is too hot and they did not use it at all (Hello, Swiss guys!). Other people also like to crank it up for some minutes before bed time even in the summer. We ourself have been camping in Lappland (which is on the mainland in northern Skandinavia) at minus 18°C and even the kids where comfortable. But maybe that´s not for everyone. (Pssst! Ask Mike from Arctic Campers Oslo - he chickens out alleready at -10°C (14°F) :-)) (Note from Mike: All lies! There where no witnesses!)
So, if you´re up in the arctic region and you will travel let´s say from Tromsø to Senja and Lofoten, you should be fine, even if you are used to higher temperatures from home. If you want to go to the mainland in the winter, make sure you know what you are doing and you should be fine, too. Stormy weather and -30°C (-22°F) in Lappland - at least consider a hotel room.
Too long, didn´t read: Heater with battery makes the camper warm all the time, winter camping possible.