Major Electrical Issue...

[USArmyCombatMedic]

Hey guys so I have a bit of an issue here. I started a thread a few days ago called Starts Fine and Stalls but after listening them and working on my truck I have realized that it has another major issue. I think I have the wiring messed up somehow.

So heres what happens, if I leave the truck off overnight the battery will hold its charge at about 12.7 or 12.8 volts but the problem is when I turn it on. I have to have a battery charger hooked up to it in order to turn it on. It starts and runs at about 12.7 volts, as soon as I remove the battery charger the truck will die in a matter of 5-10 seconds because the volatge drops that fast to 11.85 volts. What is causing the voltage to drop this fast? As far as I know all of the grounds and wires are hooked up like they are suppose to be. The alternator works as it shoots out anywhere from 14.5 to 16 volts and was also tested by O Reillys. Im sure this is something stupid and easy that I am just overlooking. Thanks for the help in advance guys!