Found the leak! My grey tanks are ABS, and have a 2" flange all around the top. Both sides are supported by a bracket which installs under the flange to hold everything up. My guess is when the tank was full, the weight was too much for the ABS flange which deflected enough to crack the tank just below.
It was the first time that the tank was full since we bought the trailer last fall, so its hard to say if it has always been like that. In any case, I dropped the tank this morning, filled it up on the ground to find the leak, and saw nothing. I got my DW to sit on the tank for weight, and I lifted the front left corner.......... and the crack opened up and leaked like a sieve. Stopped when I let it go.
I gouged out the crack with a hot soldering iron which did a good job closing the it, and I filled the remaining channel with two part epoxy which is compatible with ABS. When I filled the tank again a couple of hours later, the leak was gone.
I will empty it tomorrow and put a couple of straps across the it for more support and hope for the best! Hopefully they will take some load off the flanges.
Not a fun job, but better than paying a dealer way too much to do it. Sure glad it wasn't the black tank! I'm gonna put a couple of extra straps on it too since I have it open right now.