Dan,

I had a problem with this for a while (couldn't leave my van parked for a
week without the battery discharging). In my case, it turned out to be a
mis-connected wire at the alternator... the small blue wire that powers the
fridge relay (and controls the ALT light in the dashboard).

THis is easily checked by verifying that your ALT light (and O2 light)
works properly. It should:
        1. Come on when you first turn on the key, but before you start the van.
        2. Go out once the engine gets up to speed.

  If the ALT and O2 lights work correctly, you can ignore the rest of this
message. If they stay dark when you first turn the key, suspect the blue wire.

Some alternators have two spade terminals that the the blue wire can fit
onto, it should be on the terminal that does *not* show 12V when the bus is
turned off. This wire should be connected to the terminal that only exceeds
12V when the bus is running, and the alternator is spinning. If you have a
two-terminal alternator, and this wire is on the wrong terminal, it will
disable your ALT light, and *leave the fridge relay turned on all the
time*, sucking the charge out of your battery when the van sits.

Good luck,

Tim