Most aircraft manuals (either flight crew and/or engineering ) will somewhere contain a procedure usually done by maintainance personnel for draining of potable water tanks and lines etc, in the event it is known the aircraft is going to be left unattended for a considerable length of time in very cold conditions....in my experience that is generally not done for a night stop unless it is really really cold....the cheaper option then is probably to use an external heat source to keep the internaltemperatures at a reasonable level.