In the winter I put a 1kw aquarium heater in my static tank (set to 16C, above 20C legionaries can grow) and the same in my van tank. The on in the static tank is plugged into a temperature sensing gizmo (it looks like a timer plug, £25 off ebay) and only switches on when the outside temp drops below 4C. I fill the van with pre-warmed water when I get home & its still lukewarm 5 hours later, so the heater in the van tank is only plugged in last thing at night.
When the temps get serious (Cold enough to stop me working) I leave the heaters on 24/7. I'd rather pay a bit extra on the electric bill than have the hassle of bringing everything in the house or the cost of replacing broken gear once everything thaws.