Good catch on the oil pressure as GM used it to "Kill" the fuel pump circuit if you found yourself upside down or in an accident.
Great Job to "Both" who posted about bad diodes in the alternator. I have seen this before many times. Positive diode shorts to ground, Negative diode shorts to positive. Or open if alternator is weak, but not yours in this case.
Also; Quick note, but with your DMM on A/C voltage, engine running, at the alternator B+ to ground, your A/C voltage will increase with RPM's, so Reevved up a little but reading should not go over 0.90 volt A/C. This indicates bad alternator diode also. Normally 0.15 volts A/C
Question; Do you have any after-market stuff? Do you have any that was removed and wires & things left behind. CB, Amp under seat, trailer lights or trailer brakes with green fuzz "Scotch-Locks" behind the rear bumper, and other stuff like that.
"How long have you owned this vehicle? How long have you had "This" problem? What repairs where done recently? (Ask twice / answer once)
I almost 40 years mostly in dealers, but have seen factory wiring with fuse out-of-fuse-box under dash from factory. Rare cases but normally with A/C, Cig lighter (Hot All the Times) or dash (IPC). Just saying, because if there are no aftermarket left-overs, alternator unplugged & B+ off, all the factory fuses out, then it requires a short finder kit that is simple once understood.
Ironically, I have mine out last night, thinking I had a wire to solder back on, but did not. Look, normally hook to both side of a fuse that is the offender, but in your case, you have to go to the battery.
These are simply put, a "30 AMP Self-Re-Setting Circuit Breaker" inside a tube. When attached, it will open if current goes too high. In your case, this is not the issue.
The other half of the Short-Finder kit, is a magnetic sensitive gauge that will point in the direction of the load or current draw. Gauge is small about 2" and held with back to the wire. This is a great cheap tool to have anyways, since you have a DMM already.
I would make sure the battery is fully charged, hook it up, then take the gauge to any large wire (8-AGW) or larger under the hood with all fuses in place. Repeat your conditions, like key off and out. You may remove dome and courtesy lamp bulbs as well to open doors to get in and out while checking.
Once you find a "HOT" wire, the centered needle will move. Follow the wire noting the color. You will not be able to follow the whole wire, but look to see where it goes, then check it there. Repeat until the wire ends at a component.
I hate to say this, but you may dis-connect the VCM/PCM on the vehicle. It and the stop lamp circuit are hot as well as turn signals for 4-way lamp use also.
Question: Has there been recent work done on the truck?
Just tossing out some ideas. Harbor Freight may have one, but may provide the answer to what you are after. So, a lot of ideas to check. Look underneath the cab, bed and places where wires get pinched, like between the engine and tranny. Stuff like that.
Best of luck / I will follow but with all of the fuses out, disconnect everything at the alternator. I don't if a starter solenoid would cause an issue, but anything is possible. In my driveway, I would used the DMM across one fuse at a time...right there are the fuse terminals with DMM on AMPs as a Fluke 87V will handle 15 amps internally. Go for it!
