What exactly is the problem? Battery dying over night?
You can use a good DMM that has an ammeter function(measure amp draw).
Unhook the negative cable, then see hook the DMM in-line between the cable and the negative battery terminal.
So now all the current is flowing through the meter(you do NOT start the truck like this... you'll blow the meter!).
See how many mA it is drawing... should be pretty minimal... under 50mA in most cases.
I have seen alternator diodes intermittently short out/fail before... essentially they will immediately dump the battery dead in a short period of time randomly over night... Had a Ford Focus and was trying to replicate a current drain that we were NEVER seeing... well after about 4 days of checking it on and off we caught it dumping over 100 amps
The battery will discharge REAL fast when that happens. Generally if this happens, you can catch it by using a temp gun(or your hand) and feeling the alternator's power wire close to the alternator... it will be considerably hotter than anything else around it. Note: This will only be a valid "test" if the engine and everything is the same temp... like sitting overnight or something. You will also need some luck as you need to catch it while discharging or right afterwards.
Another possibility is the voltage regulator as others have stated... what that will do is power the alternator's "field" even though it's not spinning... so it will just use power trying to power the alternator's field.