Ignition Issues

[Terra Nova]

I have a 2002 410 Sports Sedan. It has twin 8.1L Mercruisers - 360 HP each. When I first got the boat in June, I was having trouble starting the port engine. Often it would start no problem but after running for a awhile and then turning off to go for a swim or anchor, I couldn't get the engine to turn. would use the generator to power up and eventually replaced both cranking batteries to be safe. turns out the starter was bad and drawing a lot of power. I took that out and had it rebuilt and it turns no problem now.

Interesting enough now, I am having an issue with the starboard engine. If we are on shore power, the engine starts no problem. If I am out for a day anchoring, the engine won't crank at all. I have to turn the he port and after about 2 mins, I can get the starboard to crank no problem. Volts on the cranks are about 12.6 volts before I start the engines which I think is enough power to start.

Any suggestions on what the starboard issue is?