The battery voltage meter is jumping from 12.6 all the way up to 15.1 while driving at a steady pace. Is this an alternator problem?
Very likely, yes.
From what you say about the issue my guess is the alternator may be fairly old and the brushes inside it are worn out. At 15 volts, that is the top end of the safe charging voltage. You don’t want any more than that and so it seems that the regulator is doing it’s job. Replacing the alternator should fix you up. You should also make sure the battery is in good shape at the same time as the two work together. Also make sure that the main connection to the alternator is snug tight and isn’t loose, which could cause this problem too.
That’s more deviation than I see on my older Corolla, but newer cars use complex charging algorithms to both keep the battery charged and power all the electricity using gadgets new cars have these days. 13.5 - 15.5 volts is the typical charging range, so unless you have other reasons to believe there’s something wrong, suggest to simply monitor the situation.
The voltage from the alternator is regulated thru the ECU.
The ECU determines the voltage required from the signal it receives from the battery current sensor.
I’d first try replacing the battery current sensor.
It’s a lot cheaper than an ECU!
The very first thing I’d do is rule out the instrument cluster
At idle, hook up your digital voltmeter to the battery posts
If the reading on the meter is jumping, and so is the gauge in the cluster, then we can assume your cluster is okay