Did you verify the charging voltage at the batteries with a real meter? Sometimes that's all it takes to see things are fine but the instrument is busted...
That said, I've never seen a 3G pump more volts at idle than running, so that's weird!
I have a hard wired digital meter in the dashboard, and the 15 year old plug in digital volt meter, and both were within 0.1v of each other. So I didn't see any point digging out a separate meter. The hard wired one is on a separate custom circuit/fuse panel from the factory cigarette lighter.
The meter there is on separate wires from the added on outlets so it gives a pretty direct reading of the driver side battery, via a solenoid that's on with the key on.
I didn't think to record a video, just took a picture. This was first thing, 7:42am
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After driving a few miles to a park (7:54am) to walk Lucy, that one was idling at 14.7 and the old plug in one was 14.6 at the factory cigarette lighter, which was the first time I checked it with the plug in, so it went down some after warning up.
Try switching various high-current loads on and off. Lights, heater ( blower ), brakes, and special attention to glow plug activity. See if you can correlate to any disturbance of the van's power rail.
The disturbance my glow plugs were placing on my van's rail was what tipped me off to my glow plug controller contacts not opening up properly. It showed up on the voltmeter.
I would do a quick clean of the battery post/connectors too. You probably know the routine...the run through with the spiral wire brush and post clean with a strip of sandpaper.
I get the idea your voltage regulator is seeing insufficient voltage sense and is responding by increasing alternator call for more voltage.
I had the headlights and blower on (about mid) because it was cold and a bit cloudy yesterday, and it didn't make a difference.
The voltage reading with glow plugs on is something like 11.2 or 11.6, i can't remember for sure but it's above 11 for sure. It was close to 32° when I started it yesterday morning and it had no problem.
I just cleaned the battery terminals and posts with a proper terminal cleaner just a few weeks ago after replacing the brake booster and master cylinder, because it had weak cranking after being disturbed. Cleaning the terminals solved that and voltages were fine when driving, 14.3-14.5 typically.
