DON - I am referring to a second hot water system like the AQUA HOT system.
Might be worth looking into for you... I first thought they were a vary large footprint but after looking at few system they come in all sizes...
Being a closed system you can fill these with antifreeze...
smkettner got me thinking along these lines when we was discussing the different available 12VDC hot water pumps on the market... He heats his trailer tanks with hot water zones.
I would think running two or three different hot water zones in your setup would be very feasible. There is all sort of different heat exchangers on the market from the simple wall mounted kick panel ducts to full blown units mounted in cabinets etc with blower etc.
Then you could run other hot water zones around some of your other tanks, maybe in the floor of the bath room, any number of other places where "CLEAN" heat could be used.
The PEX lines can go anywhere throughout your trailer.
Sure something to think about. I'm thinking this is about the lowest power consumption footprint one could think of. How hard is it to keep a tank of hot water at a preset temperature.

Roy Ken