Frank,
If you charge the battery, park the truck, and several days later return to find a dead battery, you don't need to look in any circuit that is switched...tailights, dashlights etc. Concentrate on a circuit that is energized when the truck is just sitting, the charging system, battery side of the key or ampmeter, hot side of the brake switch. Although not common, a battery can develope a shorted cell which will discharge it. Sometimes, a cursory exam of the rig will turn up a bare wire that is laying against the frame or a mechanical component.
Maybe your check engine light is stuck on.
Stuart.
