John:

I used a Coleman BlackCat a few years ago in my
'81 non-Westy in Death Valley.  I think it is
rated about 3000 BTU.  At the low 40s, and aimed
at where you were sitting, it worked.  It
certainly did not warm up the van.  This is
with no curtains or insulation on the windows.

I have a Carver in my Westy--something like 7000
BTU.  It warms up the van with the top down.
Maybe with my new fleece blanket "lid" to
close off the upper part.  Again, it helps to
close the curtains, which I seldom do.  With
the top down, it will warm up the van at 30
degrees after a while.

So I would say that 1000 or 3000 BTU is not
sufficient at 35 degrees.

                        Richard A Jones
                        Boulder, Colorado