Could be many things - grounding issue, auxiliary light that the switch isn't turning off, some solenoid that isn't turning off.
I would check your glovebox, hood light (dad unplugged his hood lamp potentially because of this very reason on his dodge 2500 gasser), cabin light, any little light that might be around.
If you don't have the time to track down all your circuits, you could potentially install a disconnect switch, however, that would reset your transmission, radio, etc - unless you bought a memory saver.
Someone else might be able to help better than me.
Could be you have a bad battery, and its drawing down the other battery.
you could check the fuses with a test light when the ignition is off. some fuses will have power for circuits that are live when the ignition is off. like the horn or dome light etc. but check them all and use the fuse chart in the manual to see which fuses you are checking. Also you could use a test light between the Positive batt post and the positive battery cable and start pulling fuses until the light goes off. Most common issues would be alarm system or aftermarket stereo memory.
If your truck sits for days without running and you have a factory alarm, that will cause a battery draw.