Sorry to resurrect this again, but I am reading posts about winter caravanning (see Winter Caravanning in Equipment and Accessories, for one), and the only advice seems to be to protect OUTSIDE waterpipes/containers.
So what do winter caravanners do about their INTERNAL plumbing to keep it safe from frost damage. They obviously can't drain down the water pipes and leave taps open,etc, on a freezing night, or all the other things that people have recommended for winter storage.
In other words, they rely on the caravan's internal heat to do the job - which was what I was suggesting I might do during cold snaps to protect from frost...(But was advised by many esteemed Forum contributors not to!!)