Van is fine, all toasty
Last year I spent a bit of time sorting out my brick-built store shed where I keep my portable, solution hoses, chemicals etc etc.... draught proofed the door & window and put 4" of polystyrene insulation in the roof. Installed an electric fan heater which runs on a thermostat, and it has been working really well so far.
Got thermometers all over the place in there... the fan is set blowing down at an angle but it's amazing how half way up the room the air is at 10 but at floor level it's touching freezing!
We're forecast -7 overnight towards the end of the week so that'll be a good test.