Eboy - Water is one of the things that can cause an aircraft damage when parking overnight. We solve that by dumping the remaining water before we land. Later on, when the aircraft is first connected to ground power heaters warm the water system, including the filler nipple, to prevent it from freezing. It definitely works down to minus 27C. The heaters cycle through various schedules all the time the aircraft is powered. The other items are the brakes, that are left off, and and the batteries. The aircraft's manuals state how the batteries should be handled although it is generally easier and cheaper to keep the aircraft powered during a nightstop.