Mike (the OP),
I assume that during cold weather camping you're going to be on hookups or, if cold weather dry camping have a pretty good sized RV battery system and a generator to keep them charged?
If so just look for an RV - or have installed after you buy an RV - 12 volt DC powered (they are available as 120 volt AC powered or 12 volt DC powered) heating pads onto any tanks exposed to the outside air temperatures. With heating pads on your tanks, all RV furnace air can be used to keep the inside of the coach warm instead of some of it being used to keep warmth around tanks. The reason I say have "12 volt DC" pads installed is so you can use them during hookup or non-hookup camping - or when driving down the road without having to run the coach furnace ... which makes for an overall more flexible tank heating system.
The slightly warmed water from the tanks lying against the tank sides of the tank emptying valves keeps the emptying valves from freezing so you can operate the valves for tank emptying during cold weather.
To my thinking, tank heating pads is a superior way to keep your tanks' contents from freezing - over wasting precious coach furnace hot air and propane to keep your tanks from freezing. Besides, electricity is easier to find or make than propane is during either cold or hot weather. Why keep a whole volume around tanks warm with heated air when all that is necessary is to keep the liquid inside from freezing by merely having an area on the tank bottom kept warm via a flat, glued on, tough rubber material, built-in thermostatically controlled ... 12 volt electrical heating pad?
