voltage while running changing over time

Seeing 13.5 at 3000 rpm, but as low as 12 at idle. Seems same for both engines. Belts seem tight. I just noticed 1 of the 2 the green lights are out on dash. Forget the lable for them but something like the "system charged". Maybe one altenater?

You should do the voltage test at the battery with a DMM, not using the dash gauge. Expect some drop at dead idle, but I suspect the "12" is really 12.5 or even a bit more at the battery terminals.

Ditto using a decent meter to measure at the battery. Specifics will vary with the particulars of the install. The standard config places the sense wire for the regulator at the starter lug - not always the best location. Also, particularly with one-wire setups, the alternator won't "turn on" until rpm exceeds idle. Finally, the wiring of the dash meter can effect what is displayed. The factory setup uses a simple regulator and its output voltage won't change like the newer multi-stage units. If you see the voltmeter deflect when a load is switched, it tells you the wiring was not done optimally (or has degraded with time).