Current draw killing battery(s)

['68OrangeSunshine]

Hi Keith,
I just got an '03 Tahoe C/1500. Batt was an EverStart with a 2/24 sticker. [but no warranty paperwork from the PO]. Truck had connection issues in the rain. POS terminal bolt has plier chew marks. Week before last it started OK with a turn tightening the terminal. Volts were 12.7. Monday tried to take it to work -- no start. Grabbed '71 Jimmy and got to work Monday and Tuesday. Weds, I checked the batt, 6.3 volts. W/O paperwork I didn't go to Walmart, I was going to use it as a core on a new batt from Interstate. The guy there said, 'let's see if it will take a charge,' so they have it overnight.
But a drop of six volts on an unused truck seems extreme. Not used to these ''New'' trucks. ALL the door locks open when I go into PARK. Window up/down switches have been playing tricks, and the DS RVM will go screwey when the rocker switch goes over to adjust the PS RVM. Pass Side mirror is stuck at curb view.
[I had it adjusted fine until I showed off my ''new truck'' to my next-door-neighbor, and he said ''what's this do?'' and poked his finger on the mirrors switch and both mirrors went nuts.]
I pulled the 15@ fuse for the RVM but that also killed the window up/down circuits, so I put it back.