I thought I'd add a little bit of data to this discussion.  My '85
adventurewagen has one of those 12,000 BTU heaters, and according to old
brochures, this was a standard feature.  I haven't owned this vanagon very
long, but I have played with the heater a bit.  I ran it for 24 hours
starting yesterday afternoon in a place that gets pretty cool at night
(upper 30's) and I set the thermostat in the upper 70's.  I also turned on
the fridge and set it to almost the max. cold setting (made a nice tray of
ice).  I didn't spend any time in the van while the appliances were on.

At the end of this test, the aux. battery needle was at the border of the
yellow and red section (1/3) up from the bottom.  Both appliances were
still churning away, but I imagine that the battery power would have only
lasted a few more hours.  However, the propane level needle hadn't even
budged.  My sense is that this heater is pretty efficient, and though it
is 15 years old, new RV catalogs show the same basic model for sale.  I
get the sense that it's one of these things that was designed well and is
a workhorse.  If I didn't have the fridge on, or I had put it at a temp.
setting more in the middle of the range, this would have added to the
battery  life.  In addition, I could have lowered the temp. setting on the
thermostat as it was much warmer than necessary.  Still, I wonder if the
battery would have lasted for a few days without recharging.  So, extended
camping without moving would require a solar panel.  The aux. batteries
charge up quite quickly, however, and the adventurewagen brochure even
mentions the possibility of idling the engine for this purpose.

In any case, based on my very limited experience, the hydroflame heater
seems like a great heater for a vanagon, and I couldn't help but think
about how nice it would be to have one installed (somehow) in a Westfalia.
I'll leave that project to the engineers among you.

David