Water coming from grey tank

[ltoews]

During this past summer, we had a small trickle of water from the grey tank (not the galley tank) running out from the underbelly. The water only leaked out when the tank was completely full. In over seven years with this Montana, we've occasionally had the tank so full that water started backing up into the shower and never had a leak underneath. I would like to know if the drain pipe from the shower and sink is cemented into the tank or is there a rubber grommet that the drain pipe goes through? A dealer’s service department says the drain pipe is just stuck into the tank. Do these tanks have a method to release water if completely full so they do not get too full and rupture? Has anyone ever had an experience like this?