if gazza reads this post he will back me up on what i write .
ive used aquarium heater in my van tank 400 ltr for the last 3 years , i gave gazza the same type of heater i use which is a 300w this will get your water to a luke warm temp but you will never miss a days work by using 1 , allso as the water heats up in your van this gives a radiator effect so keeps the rest of your stuff from freezing .
im just in the process of having a static system frame built which runs off my central heating ,it will never get my water to a boiling temp but it will never freze either .