Low temps ahead

If the bottom of the tank is exposed to the elements It should be heated if liquid is inside during freezing weather. Nothing hot enough to melt or harm tank material.
http://www.ultraheat.com/rv_products.html
Click on how to operateon above.

Here is some good reading also.
http://www.trailerlife.com/trailer-how-to/trailer-tech/cold-weather-rv-camping-guide/