Water leak, we cannot find.

Today water started leaking out from between the membrane on the very bottom of the 5er and the metal frame the screws that hold it on are attached to. Upon inspection, the membrane was full of water all the way across the trailer and the entire area in from of the first axle was sagging severely. We shut off the water spigot, cleaned everything out of the basement, there was no water in the basement, and removed the wall to gain access to the area behind the water valves. With the wall removed we inspected everything and saw no leaks. So we turned the spigot back on there were no leaks anywhere. Next, we turned on the water in the kitchen sink, bathroom and shower one at a time. There were no leaks. After that we removed all the screws that were holding the membrane on one side. Here, we could see where water had been, but couldn't tell where it was leaking. I think our next step is to remove all the screws from the membrane in front of the first axle and pull the membrane down to get a better view. But before I do all of that, I thought I'd ask all the knowledgeable people who might have been here before. Is there a better idea than removing the membrane? Can you offer me any ideas of how that area could get wet? The wet area is all below the basement floor. By the way our trailer is a Grand Design Solitude 369RL.