Although not inside the Vanagon I towed my 61 bus, full of parts and
interior pieces weighing in excess of 2000 lbs and averaged a documented
21-22 mpg over apx. 1500 miles.  That's with a 2.1l (fully balanced,
headers, intake), manual trans.

As for loads inside the Vanagon the largest was a full length roof rack for
the bus, which fit inside with the hatch closed, see pic here:
http://v-dub.musa.com/images/61Bus/Photos/April01/

The heaviest load, as I remember it, was 8-10 8ft folding tables on the bed
and the cargo floor underneath *full* of file boxes which themselves were
full of paper.  Very heavy, but an excellent ride.

Dave
251.org
87 Vanagon
61 Microbus