Some things to try.
First turn off the truck and pull the key. Shut everything off (GPS, cell phone charger, etc.) and close the door. (Dome lights need to be off.) Open the hood, if you have a hood light pull the bulb.
a) Pull the positive battery cable off on the driver’s side battery. Pull the positive battery cable on the passenger side battery. Hook up a test light between the positive battery terminal and the positive battery cable on the passenger side. You should see no light or a dim light if you have little to no draw. If you see a bright light you have a good size parasitic drain issue.
b) As long as the test light was dim or dark you should be good to go. (At least for a couple of weeks at a time.) If you want to test further you can hook up your test meter between the positive battery post and battery cable on the passenger side. Use the amp meter function on your meter (You usually also must move the lead to the 3rd hole on your meter for this feature.) You want to see something in the ranger of .5amp or less.
c) If your test light was bright or you amp meter suggested something more than .5 amps you should
i) Put the test light back on and unplug the regulator to see if the light goes out. (Bad regulator or wiring.)
ii) Then unplug everything on the alternator to see if the light goes out. (Bad alternator or wiring.)
iii) If the test light is still lit start pulling fuses till the light goes out. When the light goes out you know what part of the wiring harness is plugged into the offending device.
With that said it gets tricky with things like aftermarket stereos, aftermarket GPS's, aftermarket alarms, etc. So you may see that the fuse for the wipers is stopping the draw, but unknown to your the prior owner hooked a aftermarket alarm that is now malfunctioning. So check to see if you or a prior owner hooked something to the power feed off the offending fuse.
Let us know what the results are!