Check the voltage under typical load. Could be corrosion on connections, undersize wiring or a combination. We started bringing our boat home to MI from VA after we bought it. Started getting low voltage warning from the new VHF. Then the auto pilot started complaining about low voltage. Took a day off to figure out how it was wired. I had untinned ground bars, too small gauge wiring and too much added equipment. I found a way to make it work to get home and after we got home, I added a new power and ground along with a new power panel to the bridge dedicated to just the electronics. No more problems.