Here's something easy to check-
Make sure the glove box light turns off when you shut the door.
That could drain a battery, probably not overnight, but maybe over a day or two.
Without the engine running, the ammeter should show a discharge with the headlights on. After engine start, headlights off, the ammeter should show a charge for 30-40 seconds. You should see the needle jerk when you turn on the headlights. Lack of these indications could indicate a bad ammeter or a blown ammeter fuse (63-72 GM trucks have fuses for the ammeter). A bad ammeter would not generally cause your issue. It would either have no effect or kill the entire electrical system like a master switch, depending on how its wired.
regards,
Leon
