Got it. In my 52DB the guest head portlight was leaking when it rained and that allowed fresh water under the floor which took me almost a year to find and sort out.....I think the 400 is under the aft. Our 410 is under the front v berth. I didn’t completely dismantle the master berth yet but I don’t see anything showing the tank is leaking. As a test I drained it completely and put it on the hose and it still collected