I have rethought saying that using an external heat exchange would add complexity. That is probably wrong and it would also make it easier to ensure you are only extracting heat from the cooling system when you are at operating temperatures.
If you cut in a bar and plate or a tube in tube style (both have their advantages and disadvantages) into the heater circuit, and just pump water to the heat exchanger from your storage tank you can easily integrate a thermostat so it only pumps water when the engine is hot and as you say coolant volume remains basically the same which is desirable.
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Being that you may travel to freezing temperatures it would be good if the return line to the tank was above the waterline in the tank so that the fresh water traveling to the heat exchanger siphoned back to the tank and no fresh water remained in the lines. You would also want to ensure the heat exchanger could drain as well.
The main issue is line size. Ting into a 1.5 or 1.75" hose is a lot harder to find parts for than it is for regular 5/8" hose. The other thing is the coolant in the heater circuit is always full engine temp. The rad circuit will vary in temp and volume flowing through it.
Thanks! You are providing a lot of useful feedback/knowledge.
The way I've seen it done with the heat exchanger, is not to pump water from the tank directly through it, but to use a closed circuit with a non-toxic but non-water (glycol?) Liquid, so freezing would not be an issue I think, and if there were a failure, risk of contaminating the water would be mitigated.