This really is starting to sound like the 'age old stocker Mopar charging system' in pretty much normal operation to me.
One other thing you might check, is to kick it up to fast idle, after it's run enough to normalize the battery. Stick one meter probe on battery + and the other on the alternator output stud. Turn on lights and heater blower. Se what it reads. If it's more than about 1.5V I would look for drop problems in the charge path to the battery.
This drop is not near as important as other measurements because whatever that drop is, the regulator will be "reading" the system voltage, and trying to jack it back up if the charge path drop causes the battery to be low.
But if that drop is more, say 3V or more, then it is getting to be an issue. Once again, the path through the bulkhead, welded splice, ammeter, back out the bulkhead, etc, to the battery. And if it's been bypassed, then whatever the bypass path consists of.