I heated my own tanks, so I know what I got and how it works...
60 watts on the 20 gallon black tank, 120 watts on the 35 gallon gray tank - PLUS and this is the important part, heat trace and insulation on the drain plumbing and valves. 40 watt bulb in the pump bay, and heat trace on the fresh line coming off the pump (it runs outside on my trailer)
it will take a lot of cold to freeze a waste tank, but freezing a valve is much easier, and the valves are the weak link, much easier to damage than the tank.
to keep the fresh water system limber, a pump on a timer recirculates hot water through the cold water line every 20 minutes.
my system's good to about -14F and then exposed plumbing for the bathroom that I can't access starts to freeze up - my plan is to run the recirculation more frequently or continuously when it drops past -10F
