Its grey tank, tested it by overfilling and it poured out the top.. Gonna leave it till next year and make sure I don't overfill again. Fittings all were dry so its surely a crack or hole in the top of tank.. Belly is open and dry so I am gonna close it up. Thanks for the reply's
Your gray tank (black tank, too) isn't supposed to leak EVER, not even overfilled. You have a crack or leak somewhere and you need to find it and fix it. If you over fill your grey tank, it should overflow to the lowest fixture in your trailer, maybe your shower, or bath tub, what ever is lowest. It should never dump into the belly.
Mine leaked in same area, turned out that there was a gasket left out at the bottom of the shower to the drain pipe. I had also noticed bottom of shower was not solid when moving around also. Only leaked when I showered, guess because I'm heavier than the boss. Added the gasket and reinforced the bottom of the shower, no more problem since then.
FWIW.....In my trailer I have a "docking station" compartment. Probably a standard, but mine has a port on the bottom that is hinged so that you can feed water hoses, CATV cable into the connectors. Anyway, the seal on that bottom port broke free of the bottom of the compartment and any water dripping from the city water port, black tank flush port would drain through the seal opening and into the underbelly.