There are 2 tubes side by side under the shelf that the shower sump sets on. I've snaked both sides and have only been able to get through twice. One of the times the snake came out of the other tube, as mentioned, they are side by side. This leads me to believe there's a cavity somewhere.
I can shoot water into the 2 tubes draining into the engine bilge but nothing drains back out, inter in fill up the cabin bilge.
I'll keep you posted.