Check your alternator. Sometimes if it needs a rebuild, it will drain the battery. Also, check the battery cables themselves to see if there are any nicks or cuts in the rubber coating. That can cause it too.
Also, it might just be a dead cell in the battery. I've had that happen. Take the battery down to WalMart or an auto parts store and have it tested. Could be it needs replacing.
Might be something else; these are just things that come to my mind at this moment.
Disconnect the negative battery terminal and put a test light in between the battery and the cable. Make sure it stays in contact, you may have to tape it in place. Then pull one fuse at a time until the test light goes out. You will have to make sure the dome light stays off. If your truck is a 88-98 full sized pickup you can take a piece of card board and jam it between the dash and the a pillar to hold the button in. other models may need the switch removed to turn it off.
once you know which circuit has the short you will have to check out every thing on that circuit. I would recommend getting a repair manual with a wiring diagram to be sure you know every thing on the circuit.