First thought that comes to mind is replace the battery.
Second thought (if you want to test things) is to leave the battery in the box over night with one cable disconnected. See if the battery is dead in the AM.
a- battery dead, you know the battery is the problem. b - battery starts truck, you know that something int he truck is draining the battery.
Third thought, clean the battery terminals. The screw things int he middle, push them out of th eplastic cable ends, and wire brush the screw things. In addition to the ends of the cable, and wire brush the round lead contacts at the side of the battery.