I know this going to sound like it's reaching a bit, but read to the end before bursting into laughter. :-)
What about using the water tanks as heat sinks, both the grey and/or fresh water tanks could be used with heat exchangers installed. I know there are some folks that don't use their trailer water systems for drinking anyway so the lack of cold water wouldn't be a problem and what about that ability of having your water preheated before heading to the water heater.
With another heat exchanger for heating the radiant floor plus one of the inline heaters as a backup/booster, just think of the possibilities.
Further disclaimer here, it is late, and I didn't get to contribute all day, even if I did get to read the posts. :-)