We have used the most reliable cars for the task. There where only one choice. The legendary TOYOTA HIACE with 4×4 drive and a diesel heater system.
The driver seats and the sleeping/living area are separated with a wall. After driving you must go out of the car and enter the living area thru the slide door on the right side.
This wall makes the volume of the living area much smaller compared with traditional campervans. And therefore enhances the need for external heat dramaticly. This comes in handy when the outdoor temperature goes below -10 °C. The heat from to persons and a small stove will easily change the temperature inside so that it will be comfortable.