But it was still an amp gauge, shunt or not when his truck was new not a volt meter

 

My point being the amp meter or shunt go between the alternator and battery. So if there is an issue there it won't charge the battery.

 

I thought that after they went to the shunt setup that was less of a problem. I do know I have seen more of those trucks where the charge gauge didn't work than those that did.
Check the black plug at the back of the engine compartment, if a slant 6 it's right off the back of the valve cover, I think it's like an 8 connector plug where the engine (itself) harness plugs into the rest of the under hood harness, the alternator wires go thru it, and I've seen those burnt before.
I just went thru a harness for my 85 and that was BBQd.
I went to the junkyard and got a better harness to start with and draped it across the kitchen table and completely untaped it and went thru it to check for damage with a factory service manual wiring diagram open right next to me, one thing is that is a lot of current for the size of the wire and for the size of the connector within that plug that it all is sent thru. Something I saw on another Dodge site was the idea of adding a redundant 8 ga wire right off the alternator hot post and run it across right to the battery "hot" post as well as leaving the original one in place too. Put a maxi fuse holder in this new line (that site suggested one fuse at each end) in case that wire somehow gets rubbed thru, etc...

One thing I did on that 85 was to incorporate a fuse box full of maxi fuses thto that rebuilt wiring harness where I cut every fusible link out and put it thru the new fuse box instead.
I had contacted Littelfuse company and they sent me a service bulletin about the development of the maxi fuse and how it was exactly developed to replace fusible links. I'll have to get on the home computer as I have that bulletin saved there.

 

I know when I had my 79 ramcharger and I thought it was a great idea to swap the brackets and 100amp alternator onto it from my 76 cordoba the first time it "needed* all the power it could provide (I was giving someone else a jump) I had a smoke show in my dash and it didn't start the next time I shut it off until I spliced the 2 amp meter wires together.((yeah 79s still had those)
It was after that that I found out that vehicles with the 100a option also had a little different wiring harness originally.

 

Update on all this: I took it back to the mechanic and put the starter to the test. He tested and said "starter." So I got a NEW starter (my father-in-law had aggregated a couple over the years but I am not taking any chances anymore) from AZ. Much faster start and runs normally now. I even fixed the brake light connector for trailers (at least I hope). Thank you for the help.