Joining this in mid-thread as an interested onlooker.

Was working a similar problem on my van recently. I'd see a difference
of 1.5 volts depending on where I tapped into the circuit, the ground
location, and load in the system.

That's the one item, load, not mentioned in the previous posts. When
I'd fire up my gps, laptop, and cell phone charger the voltage on that
circuit would become 12.2 volts while it read 13.5 at the battery.

Be thinking of what load you are drawing as the voltage will drop as it
pushes more amps down the wire.

MAH