Sleeping in a minivan when it is freezing outside is not as hard as you think. I have camped in my Odyssey down to 15 degrees Fahrenheit and stayed comfortable. The key is preparation and the right gear. Here is exactly what I use and what I have learned from doing it wrong.
Insulation First
Before you think about heating, think about insulation. A minivan is a metal box with thin glass windows. Heat escapes fast. I made window covers from Reflectix insulation, cut to fit each window. These block the cold and also provide privacy. For the floor, I put down a thick rubber mat and a layer of carpet. The difference between bare metal floor and insulated floor is at least 10 degrees.
Heating Options
For heating, I use a diesel heater. These are small, efficient, and safe for use inside a vehicle. They run on diesel fuel and produce dry, comfortable heat without condensation. A typical diesel heater costs about $150 and can run all night on less than a gallon of fuel. Installation requires cutting a hole in the floor for the exhaust, but the result is worth it.
If a diesel heater is too much work, a 12V electric blanket is a good alternative. It draws about 50 watts and will keep you warm all night. Pair it with a portable power station for best results.
Sleeping Gear
Your sleeping bag should be rated for at least 10 degrees below the coldest temperature you expect. A -20 degree bag is not overkill for winter camping. Add a sleeping pad underneath for insulation from the cold floor. A wool blanket on top of your sleeping bag adds warmth without bulk. I also wear a beanie and thermal base layer to bed.
Ventilation
Even in cold weather, you need ventilation to prevent condensation. Crack a window or install a roof vent. The moisture from your breath will condense on cold surfaces and make everything damp. A small battery-powered fan helps circulate air and reduce condensation.
Start with a sleeping platform and add a 12V system for a complete winter camping setup.