Well, I just thought of a show-stopper. Yes, I think I can get this to work, based on what I have read above. And by the way, I have been doing a crude imitation of this project by running a tube from my sink faucet through the hatch to the outdoor shower and out to the fill port for the fresh water tank -- I just turn on the hot water and let it flow for a while. (This has worked (obviously while we are parked!) in temps down to ten degrees above zero Fahrenheit.)
But here is the deal-killer: I realized that if I were to try to use my proposed "automatic" system for boondocking, the water pump would be cycling on and off all night long. Even though I have installed an accumulator and have isolated the pump as much as possible, it would still wake us up from time to time -- the trailer is very quiet at night.
So I don't think I am going to pursue this particular option. My fallback is to check out the various electric tank heater options and see if I can power them with my batteries without draining the batteries too much. I have to figure out the minimum wattage that will still do the job and then calculate my battery capacity and my ability to recharge.
If I install the smallest possible heater under the low point of the fresh water tank (which is the Achilles heel of the whole system) and then install very thick and tight insulation, that might be sufficient.
Probably there is no way to know for sure until I try it out in the real world.
