I have experienced the same issue, and having researched it on this (incredible) web site, plan to place the second vent this winter. In my case, a water lock occurred in the vent line, leaving no where for the gas fumes to go, expanding the tank and pushing water back toward the bowl. My vent had been relocated to the deck, next to the pump out fitting, using a standard vent mushroom like we have for the fresh water tank.
Interestingly, my neighbor's Sea Ray powerboat suffered the same consequence caused by the high velocity flow of water down the side when under way.
