Keep it as is
Walt,If I'm reading you right, you actually have a very good system already. I believe your sink drain shares the raw-water intake thru-hull of the head, right? That is done by design.After you're done using the boat, you can close the thru-hull, fill your sink with fresh water and do a final flush of the head with that fresh water. This will help to keep your head healthy & happy. Most odors from raw water intakes on the head are actually not caused by sewage, but rather by sea life (shrimp, small fish, etc.) that die in the intake tube while the boat isn't being used. By doing the "fresh water flush" you'll be doing everything you can to keep odors to a minimum.Also, my sink drains directly to a through hull below the water line & it causes no odor problems whatsoever.As for connecting your head to a fresh water source, that is a very bad idea. Even new heads will leak a little sewage from the "outgoing" waste water to the "incoming" flush water. If you're using raw sea water as your "incoming" source, you'll be ok. However, if you're using one of your water tanks as the "incoming" source, you will contaminate that tank with sewage causing a horrible, unhealthy condition for your washing/drinking water. Even if you had a dedicated tank for flushing water you'd still end up with a contaminated tank that will eventually be incapable of holding clean water for your flushing needs.You've got a good set up there. I'd keep it as is.Hope this helps!!Allen Schweitzers/v FalstaffC-30 Hull# 632