If the battery only discharges while the truck is idle - and stays in a charged condition while the alternator is operating - that would kind of eliminate a blown diode set.
There is one hot lead that does not pass through the firewall junction block. It runs straight from the starter to the cable transit. On the cab side of the transit it splits off and heads in two directions.
One lead runs to the far left bank of the fuse block (the BATT group) - if there is a clock installed it plugs in that bank. So does the DOME/CARGO lamp circuit. There could be something in either of those circuits that might pull about 2 amps - and yet not blow a fuse. Try pulling the plugs out of the sockets and watch your amp draw.
The other branch of that battery feed goes to the headlight switch. Except for the actual head light lamps, all the circuits that are supplied by the H/L switch are - at some point in their run - lead through the fuse block. So presumably pulling one fuse at a time would have detected the phantom load.
That leaves only the headlights and associated wiring. That circuit is protected by an internal circuit breaker within the H/L switch. Problem is - it doesn't open unless the current exceeds 6.5 amps. Your drain is not great enough to trip the breaker - and there is no fuse to pull. You might want to just pull the harness plug from the switch body and note whether the drain goes away.
You must be registered for see images attach
You must be registered for see images attach
You must be registered for see images attach