Thanks for the replies and help.
Alan; Good points. The water fill hoses are all tight and not leaking. I had just re tightened them and ispected them (they are new). the vent hoses empty into sealed containers in a locker, no water there. No leaks
at any of the pipes exiting the tank. Besides these pipes are no where near where the water is collecting and with not a very good way to get there.
The problem in finding the leak point is that the tank is molded into the boat and under the settee seats of the dinette. However the tank does not comprise the "outer vertical wall" that makes the kickboard of the settee seat. But it is quite close behind it. So finding the leak will entail either some telltale sign from inside the tank (very difficult) or cutting access holes in the settee seat vertical wall at likely places.
So while it may seem that finding the leak would be the logical first step. It actually might be easier and more effective to just effect a repair to the whole tank. The tank is also partitioned and while there are two access holes, quite large ones, still the far end of the tank is barely reachable with an extended arm much less being able to see in there. (these are big tanks they are essentially 7' 5'' long and about 32" wide and varying in depth
from about 2.5 feet to approx 18") the 18" dimension is of course the hardest to reach...and they are curved!)
So I can spend hours cutting up the settee in a very likely futile attempt to find a crack that is leaking 2.5 gals/week. or I can spend hours fixing the inside of the tank... I think I need suggestions on fixes that are compatible with drinking water
The epoxy seems a very good candidate...any naysayers out there?