I let the van interior get cold at night. Depending on how you use the van, what I do might be an option. I use a 12 volt DC $30 rear seat heating pad under the sleeping bag. Works fine to keep body warm at night but my head was cold. On extra cold nights I wear a balaclava to keep the head warm. Seat heater has 3 settings: off, low and high. Low setting was too warm so I installed a recycle timer so seat heater is on 10 minutes and then off two minutes. No noise.
Bought Transit remote start so I can warm van before getting out of bed. Set dash controls for max. heat and select my 750 watt electric air heater that runs when engine runs using the vehicle powered inverter for power.