Does te inside ofhe 'van need to be heated? I believe that as long as it is dry and ventilated with all water drained it does not matter if it is cold inside.
I hav also been advised in the past that heated air will condense on a cooler surface ie the inside of windows, mirrors or worktops etc, this condensation is water which will lead to dampness so again I keep my van cold, dry and well ventilated.
Incidently I also keep the mains hooked up through the winter, this keeps the battery charged and add to security bcause the lead would have to be cut before the 'van could be moved away from my house. I hope that anyone who cuts through the lead des not get an electric shock (NOT)