Why does battery dies when truck sits for a couple of days?

have the battery tested (high rate discharge test and or hydrometer test)an inductive amp meter would be needed to see if there is a parasitic draw on the battery (something staying live when the ignition key is off).

Since all batteries create power by a chemical reaction, we say that the chemicals have exhausted themselves in the reactions. This would mean that a dead battery is exhausted. --------- lightdeal