John,
The vent pipe (1.5") that goes up through the roof is glued at the tank. I've never had water come out my roof vent on a gray tank but I have had my shower fill-up the tank for the upstairs.
I have had brackish water come out of my black tank roof vent in my previous demo coach. Brackish water running down the outside of the coach. Yuk!
To my knowledge, there is no other vent on your gray tank. My guess is the tank or the vent connection at the tank has been compromised by the pressure from overfilling for a sustained period. So they tank relieved itself there instead of being able to build enough head-pressure to push it out the roof vent.
That said, if the vent joint on the tank is the issue, you may still be able to use the tank until you can get it repaired. Just don't fill it to the top.