In addition to the furnace ducts as mentioned, with any volume of water in the tanks, it will take a couple days to freeze given the insulation. Even if you do not use the furnace, say you use a portable heater, the conduction down to the tanks will increase freeze time to several days. Eventually the cold will catch up to the tanks with no furnace heat. You can add hot water to the tanks every once in a while also. Also a good idea to wrap the dump line with bubble wrap to reduce cold conduction through the pipe for extended cold.