Thataway-
Bob, that's the great conflict some of us have with these design problems, of course, which is the natural nerd tendency is to find increasingly complex solutions using technology. However, the KISS Principle and the size of a C-Dory demand that we find a simple, elegant solution that fits in the available space. The Eternal Struggle! You are totally familiar with this yourself, of course!
I've often thought of using a fairly large tank over the propane cooktop as a solution to some of these problems.
Fill the tank with water at night, heat it up warm enough on the stove to keep the cabin warm during the night, and still have enough warmish water to shower with in the morning.
During the day, the water could be circulated through solar panels in the roof, minimizing the propane fuel needs later.
Insulation around the tank would hold in the heat, and a radiator, either passive or actively driven with a pump and fan, would distribute the heat.
The system could be wall mounted (inside the cabin of course to contain the heat where it's needed, and low for center of gravity considerations), and the water circulated up to and through a tubular heating coil element over one burner.
The same coil could radiate heat at night with the burner turned off. A computer fan could assist heat exchange and circulation.
The water would add weight to the boat, but would also add extra volume for showers until it was consumed and not replaceable from the main storage tank.
The burner could be used at night to boost heat reserves, and would require a fume/exhaust hood during operation.
Personally, I'd be glad to give up one of three or four propane stove top burners to a cabin and water heating system.
This isn't a finished concept, but the basic ideas are there.
It's probably not all that different from some heating systems used in domestic ssituations these days, only simpler/cruder.
Joe.