Dave, 

Excellent report on your system.

I forsee a possible problem with this design for the hot water system in
winter weather. Basically, it seems like water will lie in the copper pipe
that extends from the drinking water tank down to the heat exchanger. When
parking the van in the winter time at sub zero temperatures, this water will
freeze and expand, possibly rupturing the lines.

You can get around this by not having any water in there in winter but then
you lose the use of your water tank in winter weather. 

>From my point of view, the one of the beautiful things about heating these
water tanks is so the van can be out in zero degree temperatures for a week
at a time and you can prevent the water from freezing in there.

derekdrew@aol.com

P.S. Also, did you check on whether this probe and aperatus you leave in the
tank are safe to have suspended in the water you drink? --just a thought.