So I have the 2007 338 and it had the grey water system when we bought it. My dealer disconnected the Grey Water plumbing so the gallery and head sinks drained to the shower sump as you describe. A few years after, I added a direct overboard drain for the galley. That really reduced the mess with all the food particles from the galley clogging up the shower sump.
The grey water tank sits center on the boat under the mid cabin and there is no drain other than the pump out hose. Our filler caps were laid out as gas and grey water pump out starboard and gas and black water on the port side. Our fresh water filler is near the front of the boat on the starboard side.
My head switch panel is as describe with just the 4 switches. No idea what that other switch might be?
Not sure that helps. You sure have a beautiful boat.
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Jnizi
2007 Vista 338 "Better Yet"
Previously:
2007 Vista 258 " Just Right"
2005 Horizon 230