I had the same idea about buying a fan heater but the guy at B&Q warned me against it and said he wouldn't want to keep one running all night in a van.
Instead I bought a 200W Frost Protection Heater foraround a tenner. They're designed for hanging on the wall in sheds, garages, etc and it's pretty hot. It's actually designed to prevent frozen pipes so I'm hoping it'll work in the van.
Also, I connected my battery his morning to see if water was flowing and left it connected for only ten / fifteen minutes but there waqs a burning smell in my van and I think my pumps buggered. I love winter don't you.
I'm assuming a new one is easy enough to fit isn't it
