This thread comes up every year about this time.  I like to camp in the
winter and have tried a number of heaters.  Here's one guy's experience:

1500 watt ceramic heater- by far the best dollar value- usually around $20.
 5000 btu output, no condensation, takes up little space, but of course you
have to have electricity available.

Platinum Cat- The only catalytic heater I'd recommend because it is the
only one that is vented.  No condensation, tiny fan uses minimal battery
juice.  The 3000 btu size is too small for serious cold.  The 6000 btu size
runs a little under $400.  Drawback is that is takes some fabrication to
install and takes up plenty of space.  You can, however, put it at an angle
behind the passenger seat in a Westy and STILL turn the seat all the way
around. Comes with a thermostat.   See
www.geocities.com/yosemite/gorge/1706/vw.

Propex (now Carver)- Very pricey (I think Steve Lashley is getting around
$700 for them?) but a fine heater, installs under the rear seat and 
doesn't take up much space there.  Uses a heat exchanger so no fume or
condensation problems.  6000 btu output.  It does use a fair amount of
electricity to run the fan, much more than the Platinum Cat.  With the Cat
I didn't even bother to install an extra battery, but the Propex or Carver
will definitely require an aux. battery.  Noisier than the cat, too- but
that out-of-sight installation is nice.  Comes with a thermostat.  

Even 6000 btu is small for  20 degree F or colder.  When it is this cold, I
don't pop the top and this makes a big difference in the heat needed to
keep the camper tolerably warm.  

Bill
St. Louis, MO