Concept for cold weather use of rPod water systems

Obviously, a heating system would be needed for the fresh, black, and gray tanks and their drain lines. And also, it would be necessary to maintain the temp above freezing in the rPod interior as well. That's easy, there is a furnace for that purpose when off grid and a cube heater can be used when plugged in.

The typical way to heat the tanks in the RV industry is to either enclose and insulate the bottom of the RV and blow warm air through it to maintain tank temperature, and/or to use heating pads for the tanks and drains. I don't want to do either of these things for two reasons. First, electric elements are out for me because I rarely if ever camp with hookups and electric heating uses far too much energy for off grid use.  Second, I think it would be very difficult to create an enclosed insulated space under a pod that was both sealed against external air leaks and would also have enough space around the tanks for warm air circulation. 

So, I want to instead heat with hot water from the water heater. The heat source can then be propane while off grid and electric when plugged in at home, as that is already in place. My plan is to install a small 12Vdc circulator pump between the hot and cold water lines, as is done all the time for instant hot water systems. The circ pump would provide a small pressure differential on thermostat demand to heat the tanks via the hot water lines, with return via the cold lines. As a bonus, the pressure water lines would also stay warm just as they do in a home instant hot water heating system.

To transfer heat to the tanks and drain lines I would run probably a 3/8 copper line inside each drain line from directly behind the dump valves up to the elbows where the drains enter the tanks. That would provide for direct heat exchange to the water in the drain lines which would in turn circulate by natural convection (thermosiphon) to the tanks. I would change out the 1 1/2 in gray tank dump line for a 3 inch one while I was doing this both because the small pipe is I think too small to support a good thermosiphon loop and just to have the two heat exchangers be identical. Besides, that line drains too slow now anyway.  I would just make up two new complete drain assemblies so I wasn't fussing around with stinky old drain lines. The copper lines would be placed against the upper inside surface of the drain lines both to enhance the thermosiphon action and to not interfere with solids drainage. FR conveniently provides low point hot and cold winterizing drain lines adjacent to the two tanks to connect the copper lines to using pex. 

The fresh water tank would need to be done differently both because the pump feed line needs to be protected and because there is no large opening to connect a thermosiphon to the bottom of the tank with, so for that tank I would use a thermostatically controlled 12V solenoid valve and simply feed hot water into the tank drain line when the tstat called for heat. Very little hot water would be needed for this, and the feed line to the pump would get warmed simultaneously whenever the solenoid valve was open to replace the water entering the bottom of the tank. 

Of course, I would still have to insulate the bottom of the pod and the drain lines to reduce thermal losses but I could just use a closed cell foam spray kit for this as I'm not trying to make an air chamber.  I would also install cutoff valves for the outdoor shower to winterize that, I don't see taking outside showers when its freezing out 

The spray foam kit would be most of the cost for this mod because all the other parts are simple plumbing parts and a low cost water circ pump and solenoid valve. The little 12V digital thermostats (I would need 3) are about $10 each on Amazon. I think the whole thing could be done for probably less than $500 and would greatly expand the utilization of the rPod. 

If you got this far thanks for taking the time and I'd appreciate any constructive feedback.