Insulating the walls and roof was the first step towards working in my mobile garage without freezing.
The temperature has constantly dropped for the last couple of days. Cold air was streaming through several holes and cut-outs in the floor. After fixing all these holes with some remaining polystyrene insulator and sealant, the heat of my electric heater kept the van warm inside.
The heater can be operated in two modes: 800Watt and 400Watt. On cloudy days, I couldn’t use the heater for much longer than one or two hours.
Installing a diesel heater
Permanently heating the van in the winter with solar energy didn’t seem scalable. So, I extended my setup with a diesel heater connected to the van’s tank. Placed right behind the driver’s cabin, it sucks cold air through a radiator and blows hot air into the back of the van.
The diesel, fresh air for burning it and the fumes go through a cut-out in the floor.
This setup prevents the heater from consuming the oxygen inside the van,
and makes sure that the fumes stay outside.
The 2000Watt diesel heater is a perfect addition to my electric heater. It does a great job heating up my van. Within a few minutes, it was 25 degrees Celsius in my van and I had to take off my jacket. 
Next up, I’ll be insulating the floor to make the heat even better. 