Alternator Not Charging the Battery? Could be wiring issue?

[Rosie47]

Hey all - a friend and I are trying to get an '88 ranger 2.9l V6 up and running. Main issue is that the truck won't always start and won't always stay running. The truck starts right up when it is jumped and I've run it for up to 40 minutes (with the radio/air/lights on, putting a load on the battery) before it shuts off, but it will always get to a point where it shuts off and wont start back up again without being jumped (it always starts when jumped). Heres what we have tried so far:

-Cleaning up the terminals/cables/grounds from the battery.
- Tested the battery with a multimeter. Battery reads fine when truck is off (and has been jumped) and manages to stay above 13 v while the truck runs with the lights/radio/air on for a little while and then slowly drops down below a 12v reading and eventually dies.
-Bench tested the alternator (shop said it runs fine/high)
-Swapped out the voltage regulator. The alternator is not the stock alternator and the voltage regulator is external. Put a new one on there, but it still doesnt work.
-Checked all the connections going into the alternator (peeled back the wiring to check for corrosion and also tested that there was a current going through the main plug that goes into the alternator itself.
-Tested all the fuses (all the fuses in there were in fine shape except one, swapped that one out but it didnt fix the problem)

I guess it seems likely to be some kind of wiring issue, but also not sure if there is something else I could be missing.

Anyone else ever had a similar problem?