"On cool mornings, we boil a kettle of water for coffee/tea.  Does a good
job of warming up the van.  Costs us the amount of propane needed to boil a
kettle of water.  And, we have coffee and tea."

Know what you mean as I do the same, but .... the downside of that as a
"heater" is the condensation
that migrates to all corners of the interior of the van. Over time that can
have a very contributory affect
on the development of rust that you can't see, behind cabinets for example.
My preference is Propex
and that is very effective at heating and drying out the interior from human
respirations over a chilly
night.

Bob