I just run a small oil filled heater in the back of the van, I also have a Mr Heater that runs off propane and use that through the day. I've never heated water in 9 yrs. If it's that cold, I tend to have a day at home.
Rob
so you have an heater in the van while your out working in the day? interesting! have you got a link for the day time heater? thanks
my van has a heater built in on the dashboard, I turn it on when I start the engine in the morning

but seriously insulate behind the ply in your vans and inside the roof with foil bubble wrap or something, it will be warm enough all day.
Cant run a lead out to my van so never heated it overnight, 650ltr of water never froze on me yet in 8 years, fill up in the morning if its a worry.
All that stays in my van is tank , pump, hose reel and poles and its quite easy to protect them from freezing either by draining them (and setting everything up quick release - no jubilee clips) or priming with antifreeze overnight.
Simon