There have been some Sportsmobile people who have even gone as far as having a fresh water recirculation line that came out of the water heater, followed the pipes, circled around the tanks, then emptied back in the FW tank. Of course, I would bet it would be a PITA to drain for winterization, but it is an innovative concept.
I also have seen European hydronic systems from Espar and Truma that can heat water, coolant (so one can have that circulate near the tanks and not worry about that closed loop freezing when the van isn't used), and air at the same time.
