Why is my voltage gauge showing low when battery is charging?

Here’s what I know:

2001 National Surf Side 3311 motorhome. It’s on a Ford chassis.
Ford V10 Engine, Ford gauges, etc.

Near as I can tell it has a 3G Ford alternator.

While driving, the voltage gauge on the dash shows ~8V and doesn’t move much up or down. Every other gauge functions as normal (oil pressure, engine temp, tach, speedo, etc).

Testing with a good Fluke multimeter: At the battery it’s showing 12.6V while sitting and showing ~14V while running (so I assume alternator is charging).
At the alternator, I have 4 wires:

4 gauge black/red that shows constant 12V

Then a 3 pin plug that has the following wires:

Red/lt blue with 12V constant

Lt green/red with 13+V when engine is running

Brown/black wire with 7.3V when engine is running (this wire comes out of the 3 pin plug and goes back into a single pin plug that’s also on the alternator, it doesn’t go anywhere else.)

My question is should the brown/black wire show a higher voltage? If the battery is charging then what would cause a low voltage on the gauge? I am assuming the voltage regulator attached to the alternator is bad but thought I’d check here first before just buying parts. Thanks in advance.