Try disconnecting a battery cable when you park the truck and see if the battery will hold a charge that way. There's got to be a slight parasitic drain somewhere if you can maintain a charge by interrupting the circuit.
I like to use an ice pick type test light to check for parasitic drains. Connect the light between a disconnected cable and the battery post it came off of. If the light glows, even dimly, there's a small drain somewhere. Start disabling circuits such as the ignition switch, light switch, generator or alternator, voltage regulator, etc. until the drain goes away, then do an in-depth diagnosis of the circuit with the drain.
Jerry
