Every winter I think about getting an immersion heater element and a timer and rigging up some method of fitting it into my van tank.
Wrap the tank in some form of insulation, set it to turn on for a couple of hours before I get up in the morning and see what happens
An immersion heater element is cheap, as is a timer and insulation, so it's no big loss if it doesn't work out.
Maybe I'll give it a bash and see what happens....
Ian