> > electrical - 12V
> > crystal energy cell
> > 300watts/1200BTUs
> > 102CFM
> > draws 25 amps
> > fan and heater
I sure wouldn't run this very long at all.  In fact, I think your battery
will agree with me because it won't run it very long at all either.
What I can say is this, like it or not ... my little fan heater which pulls
air from the fridge did keep me warm (and my wife, who being the disbeliever
that she is, slept on the far side) through the night right outside Calgary
2 nights ago.  Of course, it was not all that cold at Calgary 2 nights
ago but it did work.  The only problems we have are this now:
(1) we need to find a way to attach insulation over all windows
(2) we need to find a better way of keeping the old sleeing bag hanging up
to make the sleeping area as small as possible.
(3) It would really be nice if we could control the fan's speed according to
the temperature of the exhaust vent.
(4) while most of the noise from the fan has been shorted, there is still
a bit which interfers with Canadian Televison (which, incidently, had to be
changed due the the "American Thanksgiving Holiday" ... should Congress 
introduce legislation requiring Canada to have the same Thanksgiving?  (no) )
When I compare the amount of comfort which I had using this method to other
methods, I have to say you ought to use my fan suggestion!
Note: My wife and I do have good -30 sleeping bags, but I did not even have
to zip mine closed.
I'll have a note on Canadian Customs & vw vanagons later.  Most scary!
Stuart Hutton