Some ideas
Congratulations on the boat!
Are you finding the water behind the seat backs or under the seats? I don't know exactly how the wood strips behind the dinette shelf are attached to the hull, so there's a question there. They must be on firring strips or standoffs of some kind, so water might go down unnoticed against the hull. There must be a way for rub rail leaks or for rail jib track leaks to get down behind the dinette. I've just never been back there.
If you have a zippered headliner above the shelf (under the side deck) you should check above there for wetness. There's an inboard jib track (on deck) against the house. The U-bolts for the shrouds come through in 8 places. Stanchion bases...
Forward or aft of the dinette, there is nothing to prevent the water from trailing aft or moving forward and being found under the seats. I think. But I'd be surprised if water moves forward or aft very much unless it goes right along the underside of the deck.
In the head, through the storage doors, you can get to most everything on the underside of the deck for a couple feet. Bring a light. There is a waste fitting on deck just forward of the dinette. And a little cowled vent for the holding tank just below it - going through the hull. Forward of the shower is a problem for access - I haven't been there yet either.
I think there's a zipper to get into the headliner in the nav station. That area doesn't seem to me to be a likely source.
That's about all the easy checks that I can come up with (if the zipper pulls aren't corroded - but there's a handy, helpful thread on that, too). I hope you find an easy repair.