Gray water

In my experience an overfull grey tank will just back up into the shower drain....

YES it can leak

Drains from sink/shower go into top of tank as does the vent pipe
Some RV MFGs glue the pipes to tank fittings
Some RV MFGs use 'grommets' that the pipes slip thru......this type allows for slight movement of tank/pipe but can leak if tank is overfilled

Shower is typically the lowest point so when trank is overfilled it should backup into shower pan.
But that also requires tank level to be high and if grommet is leaking water would come out from there

Also there is the possibility of tank crack

Ed_Gee wrote:
In my experience an overfull grey tank will just back up into the shower drain....

True.

X2, Ed Gee

No water in shower, noticed a leak near the handle you pull to drain tank. Had been camped for 3 days. Bottom is enclosed so could not see at the time. Seem to stop within minutes after tank was drained. After our recent snow and freezing temps here in Texas there were no leaks when water supply was turned on and no sound from a water leak. But maybe it had to soak thru insulation before leaking?
Thanks for the comments!

Only way to know for sure is to open the belly and watch. There have been reports of leaks around fittings and leaks due to cracks. Whatever, it shouldn’t leak.

Old-Biscuit wrote:
YES it can leak

Drains from sink/shower go into top of tank as does the vent pipe
Some RV MFGs glue the pipes to tank fittings
Some RV MFGs use 'grommets' that the pipes slip thru......this type allows for slight movement of tank/pipe but can leak if tank is overfilled

Shower is typically the lowest point so when trank is overfilled it should backup into shower pan.
But that also requires tank level to be high and if grommet is leaking water would come out from there

Also there is the possibility of tank crack

This is also quite true. Depending upon the connections to the vent and the drain at the top of the tank, it could easily cause a leak if the connections are loose. I had a similar issue in my past coach and finally just learned to live with it and ensured I never overfilled the gray.

Question: Is the drain handle pretty easy to pull to open/push to close? There are four bolts around the gate valve. These apply pressure to keep the o-rings from leaking. I've noticed that if those bolts get loose the gate valve slides easier.
Either way the first thing I would do is open up the belly so I could check the valve area for your leak. If the leak is there it may be as simple as snugging up the bolts. Or maybe replacing the o-ring.

Does it contain water? Then it can leak.

Per RVIA code, the Black and Gray system MUST be watertight on all the fittings attached to the top of the tank and the exit fittings. The correct way to do a PDI (Pre-Delivery-Inspection) is to fill up both the Black and Gray till water comes up in the shower and the toilet. Then wait 30 minutes. IF NO WATER comes out on the ground, then the system passes inspection. Even RUBBER collars which the ABS pipes are not glued, will hold the water from the shower and the toilet down tube for that 30 minutes. If the down pipes are NOT watertight, then Sewer odors will migrate out the top of the tanks. Doug