Start by removing one battery cable and placing an amp meter between the cable end and the battery post. With the truck turned off you should have zero amps, if you don't have zero amps then something is drawing current from the battery. To find out what the culprit is you can narrow it down by removing fuses one at a time and then checking the amp meter to see if it goes to zero. Once you have narrowed down the fuse then you will have to check each device ( lamp light horn ..whatever) that is on that fuse one at a time until the amp meter reads zero. If removing fuses does not cause the amp meter to zero then you have either an un-fused device that is causing the current draw.
