Circa 1996 twin engine 7.3lx MPI's with two batteries and two clean battery switches (port in #1 position SB in #2), original wiring, healthy batteries and new alternators.

It seems most accessories and systems are powered by the starboard side. A few years ago I began noticing voltage draw of 0.5v on the starboard battery gauge every time I made even small adjustments to the Boat Leveler trim tabs even when on plane. I figured being the alternator age was unknown and looked original, I'd replace it but it did not fix the voltage drop. I removed the OE power supply wire to the helm switch, hardwired the switch to the starboard battery (fused), and no more voltage drop.

Fast forward to re-rigging after some restoration work and now the starboard voltage is being drawn down from 14.0v(13.8v) to 12.5v on plane when opening all 4 Corsa Captains Call solenoids! Like the trim switch, I hardwired the exhaust to the port battery this time (trying to take load off of SB). So far only tested at idle and made an interesting discovery. The port battery gauge initially crashes to 12.5v as well but immediately recovers to 14.0v (13.8v).

Any shots in the dark as to why the starboard side voltage doesn't recover through the OE wiring harness?
Clear choice would be a bad alternator regulator but remember the trim tab pump still lost voltage through the OE harness on both the OE and aftermarket alternators; however, didn't when hardwired to the battery.