I would give OFR's a try. They are relatively inexpensive to purchase (and you could get a couple of different kW sizes).
The nice thing about electrical stuff is its
controllability ... depending upon how adventurous you're feeling, you can always add timers, progammers (on/off at various times - like an hour before you usually arrive, for example) or even (various forms) of remote control.
The only snag there is that you would need to leave the mains power on (if you wanted the 'van nice and warm when you arrive).
But I would also consider a small OFR placed under the gas boiler as well - anything to prevent it freezing up!