[CoolbreezeNJ]

I'm thinking your mechanic is a hack. Completely charge battery. Crank engine and see if it doesn't go below 9.6 volts and that should be that. Check alt while running and see if is is putting out 13+ volts.

Did you go thru the entire fuse block and see if there is anything applicable that might have blown???

Go to connections for HP fuel pump and remove and then re-plug them in. With key on can you see 12V at that fuel pump?? Is the HP fuel pump even electrical or is it mechanical as in a diesel. Probably not possible for LP pump which I'm guessing is in tank .

Check connections on alternator and remove and replace them.

Start car ,go thru all the accessories and turn them on one by one and see if vehicle is maintaining a charging voltage. While doing this bring rpm up maybe to 2000 rpm and see if it is still holding voltage. I believe that these cars have a "smart regulator" from what I might have read where it is looking for the BCM to measure voltage and then re-signal the regulator in the alternator. If you have a 2nd voltmeter and can get to the fuel pump connection monitor that voltage while your doing the rest of the stuff.

I'm almost 60 yrs old. I put myself thru college working on cars and have fixed them and nearly everything in my house since. What I have learned is to be patient and collect data as I would say that "throwing parts at stuff" has only worked 30% of the time. I would not be asking anybody to check my BCM until I went thru the scenario above. I would assume the BCM is non-volatile memory. It either works or it doesn't but it would be the last thing I'm looking for and at that if the charging system relies on a BCM signal then no doubt you could see that signal at the alternator connection.

If I have time later today I'll go thru this scenario on my own car minus HP Fuel pump voltage and report back.