Something vary similar happened with my 2002 Keystone Laredo 5th wheel.
When we used the kitchen sink water would drain out from various areas under the unit. Even after removing sections of the plastic under sheathing I could not tell where the water was leaking from.
I was worried that the gray water tank was cracked but it turned out that the drain pipe from the kitchen sink sheared off where it entered the gray water tank. It entered the tank from the top so this could not be determined until the tank was dropped.
This was a bigger project than I wanted to do laying on my back in the dirt at a seasonal site. I took it to a repair shop and the cost was about $600. This was two years ago and its been fine ever since.
