Water tank leak

[kmo303]

This is a 2004 29' class C 329B. There are two water tanks. The bottom tank fills through the top tank. When the bottom is full it starts leaking. No leaks between the tanks or the fittings vent or overflow. No leaks at the water pump or the fittings. Water will start leaking from the bottom sheet metal. It leaks down to 2/3 full and then stops leaking. Anywhere I can access or see it is dry. I cannot locate any crack or the leaking area. The leaking water underneath rig runs along a metal strap at a moderate rate.

Any suggestions would greatly be appreciated.