Adding a stbd side sea water intake

Is your port side intake out the water when on a port tack?

I don't have a sea chest on JDV so I'm not speaking from experience, but here are a few thoughts...

You would have to be heeled over pretty far to expose that intake.

Adding a thruhull on the stb side would only help if you CLOSED the thruhull on the port side.

Could it be something else? Could you be getting air in from your vent when on a port tack?

On a port tack, you'll be pushed a little bit sideways to starboard. That will create a small negative pressure on the port side of the keel. Maybe it's enough that the engine and/or fridge pumps are pulling air in from the vent?

Put a valve, or a stopper of some kind on the vent and see if that makes a difference. If it's a clear hose maybe you can monitor the water level in it next time you are out.

We have dedicated intakes for each piece of equipment (engine, fwd head, water maker, and wash down pump). The engine thru hull is in front of the engine, low on the hull, next to the keel. Probably similar to yours, but a couple feet further aft. We have never had an issue with motor sailing.