Check the Front
I had a similar experience, only leaked when full. Only spotted the leak when I pulled the passenger seat forward and stuck my head in front of the tank. Leak was hiding behind the "cross member" near the top of the front plate that mounts the fuel gauge sending unit. Couldn't see it when standing beside the plane or kneeling on the wing.
Since the leak was around a screw (not the plate perimeter) I just drained the tank, removed all the screws, cleaned up a bit. I dipped the screws in Proseal and reinstalled making sure that there was enough proseal "overage" to squeeze out from under each screw head. No more leaks.
Hope you're as fortunate and do not need to remove your tank.
First Flight 9/23/16!
No paint yet. Working on a scheme with John Deere colors or accents.