I think that the answer to that depends entirely on personal circumstances.
When my van is staying still in frosty weather I have a couple of choices. Drain everything down or keep it heated, then with keeping it heated another question, with what?
Drain everything down does not suit my lifestyle. I want the van ready to go and I am dreadfully lazy so do not want the faffing around of the drain-fill cycle.
Therefore I heat it.
I prefer to use the auxiliary heater. Firstly so that at least I know it works, and I also feel it is a bit of equipment that benefits from regular use. Secondly it is instantly on tap, I an not lugging various heaters around. Thirdly I feel it is quite economical.