Something is connected. Remove one wire at a time until the draw on the battery stops. Do not cross the battery cables. Some items do not like reverse current. If you have a Voltage Regulator, it can have a stuck cutout relay. That will keep the charging system in the circuit even when the truck is off. You still need to address the fuel issues. I know it sounds like an open can of worms right now. But it can be sorted and fixed.
