I did some more investigating, you're right that the tank is in the V berth. The hose runs through the port hanging locker in the V berth, and there's a T fitting which looks like it ran up to the where the deck fill used to be. It's been filled and painted over, you can't tell where it was. I think there's non-skid over it now.
I have a couple of follow-up questions: is there a way to seal the top without glassing it in that won't make the water taste weird or leech chemicals? I have a tube of 3M 4000 UV that I've been using for deck hardware and portlights, but I don't know if that's good for this application. It looks like it was screwed down previously, some residue from a seal maybe but it was cleaned up well and hard to tell what was there, maybe butyl rubber?
The gel coat that's in there now is in good shape. Should I paint it anyway? Epoxy?
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