We took a little different approach. I put a 5 day cooler (maybe 12 gallons) behind the head in the

and then attached an outlet hose to the drain fitting. I attached the drain hose to a demand pressure pump and used that line for the 'warm water' pressure system--regular plumbing going to the shower inside the head and to the hot side of the galley faucet. The head faucet also has warm water plumbed to it. I put in a water deck fill above the cooler and attached it to the top of the cooler with a hose. On sunny days, we fill the sun shower from the warm water side of the plumbing and let it heat up, then dump it into the drain fill tube....then refill, etc. by evening, lots of warm water for dishes and an evening shower. When the sun isn't enough, we just light the Zodi water heater and run water out of the warm water tank (the cooler)--through the Zodi in the cockpit--and back down the deck fill pipe. We run that until the water is the right temperature. Cold water has it's own pump and separate cold water tank. When you use the shower, if the water is too hot, you just turn on cold water like you would at home. Of course, it means that you hear two pumps coming on as each demand system turns on.
This system allows hot water from any source to be added to the cooler. If we are just going for the weekend, I could fill the system with hot water from the house tank. I could use the 'heat the water on the stove method and just pour it into the cooler, too, but I haven't tried it.
I would love to try running engine water through a copper coil in this tank and see if that would warm the water well enough. Keep Experimenting!
SILK