The 'dead battery overnight' problem can get very elusive.  Try this one
(never solved).
After an engine replacement in my '85 Westy I drove the van home and
parked it.  The next morning the main battery was dead.  Jumped it with
my always present portable jump start battery. and drove to the shop
that did the work.  They checked out everything they could, found
nothing.  They referred me to a local auto-electric shop for
troubleshooting and fix (at their expense if caused by their work). (I
never got around to going there).  I got into the habit of reaching over
and disconnecting the ground on the main battery if parked for more than
a couple of hours. I checked the draw with my VOM, the system (turned
off) was drawing well over an amp. And when the battery was connected,
there was a loud click from something in the vicinity of the drivers
seat.  Aux battery had no problem (was on a Surepower Isolator).  One
clue stuck with me:  after the install of the new engine the shop had to
bleed the cooling system.  When I got the van back, the heater lever was
jammed to the left.  I had had problems with the heater cable
(jammed/bent wire, was scheduled for replacement) in the past.  Forced
it loose, worked as before.  Over the next few months I found that
SOMETIMES when the heater lever was moved the short would go and come.
I could tell if there was a problem by listening for the 'click' when I
attached the ground on the battery.  The night I totaled the van the
problem had not been there when I started up to leave home.  I had used
the heater on the trip, stopped for several hours (with battery
disconnected), and the problem was back when I went to leave.  I had
reached the conclusion that something under the dash, near the heater
lever, was being shorted by movement of the heater lever/cable.  Since I
rolled and totaled the van that night, I never did have the 'pleasure'
of finding the exact cause of the short.