As you think the auxiliary battery is Mickey Moused in there, is it possible
that it's connected to the main battery all the time?  That might explain
the power drop right there, as the current will flow and try to equalise the
two.

I'd be tempted to disconnect the auxiliary battery (I believe you want a
type 41 to replace it, but I think a type 42 is the same size, just a larger
capacity) and reconnect the main.  If it starts to drain, charge it up, and
disconnect all the fuses, then add them one by one until it starts to drain
again.  That way you can start to figure out what's draining the battery.

I installed a dual battery charger under the sink, on the spare 110V power
socket, so I tend to trickle charge the batteries quite often, but I'm sure
I've never seen a large drain from just sitting in the garage or going on
short runs.

toodle pip,
--
       Dug Smith         | What is the meaning of life?
(mailto:dug@dugbert.com) |  - All evidence to date
(http://www.dugbert.com) |    suggests it's chocolate.