I was winter stealth camping a year or 2 ago and found that the vanagon
rear heater puts out a lot of heat.  The engine has to be running of
course.  I got up and cranked the engine, closed off the front heater core
by setting the temp lever to cold.  In just a few minutes the rear heater
was putting out heat and lots of it.  This was with an interior van
temperature starting point below 30 F.  It sure chased the chill
away.  Then I got the stove going to make a hot beverage and that helped
too.  At that point I could cut the engine and be comfortable until it was
time to start it back up to hit the road.

Edward