There is a simple test to see if the battery is being drained at night with the truck off. Disconnect the positive battery cable and connect a simple test light between the cable and battery post. If the test light lights up then there is a drain/short somewhere which is causing the battery to slowly die.
If your test light lights up you can expand the search by going to the fuse panel. Reconnect the battery cable to the battery and removing ALL of the fuses (make sure you mark them to get them back in the right place). Once they are removed clip your test light to a known barrery hot and start touching the fuse clips with the test probe point. If it lights up you found the circuit that is draining the battery.
Hope this helps,
Mike B
