>I also really want to avoid any permanent installation, so the whole thing
can be stowed in the garage in the warmer months.

How about  4 3V computer fans, the small low-speed quiet kind, mounted on a
flat plexiglass plate which would velcro up to the skylight opening.  This
would give your positive air extraction -- just open the side windows
slightly.  You could go as far as mounting a rheostat to control the fans'
speed.  It would also be useful in the summer!  This would be good for those
of us using a platinum catalytic propane heater inside the Westy.  I'd still
use a CO detector, though.

Chris S.

'85 Westy.