Where does the truck measure its voltage from?

The truck is a 2012 VNL. The reading from the alternator is 14.2v, and at the batteries it is 14.1v, but the dash can display anywhere between 12.8-13.8v. I've tightened all of the battery terminals and the ground from the battery, as well as the starter lugs. There was one morning that the low voltage warning happened, but the inverter was still getting over 12v, and the truck showed no hesitation when it started up. Everything else works like it should, no codes or problems, it just gives a false reading. Which wire or component should I look at next?

 

I checked the voltage on those studs when I replaced the trailer supply pass through fitting, and it came back fine. I guess I'll have to check the wires coming off of them.

 

This was the 11.6 one or whatever, where the sleeper lights don't work and the LEDs on the switches start flashing. I've got my own inverter off the batteries that I wired into the power outlet, so it doesn't get affected by the truck's voltage cut out. It shuts itself off when it gets low, and it gets its reading directly from the battery.

 

Cleaning the grounds and tightening them up got me up to 13v, but cleaning the positive ends got me back up to 14v. However, the maxi fuse connected to the truck was the main problem. It ended up vibrating loose internally or something, so when the truck went over a bump, it would lose all of its power. After removing the fuse and running directly to the battery, it hasn't shut down or lost power, but I do suspect that cable end is going to need to be cut and a new end crimped on it. The voltage now runs between 13.7-14.0 without any HVAC or lights running.

I had to cut off the wires to the original fuse because the nuts were rusted to it. The one I used to replace it was a cheap one, so the plastic housing fell apart and the copper inside split. I'm also going to focus on securing my cables better so they won't move around or vibrate as much.