I know that Toyota will have plugs for different options/applications on the same model so there
might be a plug for something that was never on your vehicle.
As for the dead batteries, you are in electrical hell now. My impression is a dead short that is draining
the batteries. Since you are grounding the truck upon leaving, go over all the factory ground wires and
check for continuity and corrosion. Undo bolts and clean up connectors and grounding points. Check the
voltage regulator on the alternator, if it is not functioning then this could damage batteries too. Go over the
alternator wiring, remove all the crap electrical tape and look for damaged wire sheathing and cracks in the
factory wires. An old trick I remember being told, is look at the wires in the dark and look for arcing against
anything. Sounds like an old ongoing issue that the PO might have decided he'd had enough of.
I had an old Mustang with a forever problem with the voltage regulator. Finally some old guy in a shop
in the middle of nowhere finally found the issue.
At that point I had to pay him hourly to find it, cause it failed in the middle of nowhere.
I'd hate for you to have to do that. I hope its something that can be found by testing, but I have a
local auto electric guy here. It's not my strength figuring electrical.