Grey water leak on top of grey tank?


Hello fellow RV'ers ...
2021 Cyclone 4007. Need a crash-course on waste tanks.
Noticed some bulging in the front underbelly of our rig. The 'sloshing' sound when pushing up on the underbelly confirmed it was water. Dropped the front underbelly to drain the water and took off the basement access walls. Big puddle of water on top of my grey #1 tank with evidence that it had recently been a bigger puddle and overflowed down the sides of the tank on to the underbelly. Water had a 'soapy' smell so assuming it was drain water, not fresh or black ... also no evidence that any of the fresh water lines are leaking.

Some context ... it's normally just the two of us and we try to dump regularly since our tank level indicators have never been reliable. A few weekends ago, we had some family stay with us and our grey #1 tank filled-up to the point that the shower drain backed-up.

So my question ... when a grey tank gets full, is there some kind of overflow valve on the top of the tank or do I have some kind of defect in the tank? I can only see about 1/4 of the top of grey #1 as the rest is under the basement floor. Can't think of any other way grey water would get on top of the grey tank. Unless it's fresh water spraying from an occasional leak?

Anyone have any ideas?


Well-known member
Best thing to do would be to recreate the full tank, and then watch to see if you can identify where the water is coming from. Best case scenario is that it’s just a loose fitting that is leaking when it got put under pressure. A cheap scope cam may help.

Good luck!

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Well-known member
It could be the vent pipe to the roof is loose or has come out of the tank. Another possibility is a loose drain, look at the shower drain at the bottom of the shower pan. Our shower drain was loose and caused a small leak there.


The gray tank (at least on our Bighorn) has two holes on the top. One hole is for the drain into and one for the vent to the roof. There is a rubber grommet that is placed in the hole to hold the pipes. To my knowledge they are not glued but just pushed into the grommet. You would be surprised how much the tank swells when full and the pressure put on the connections, especially the bottom drain. I would not have believed it if I had not seen first hand. You may have a loose top connection of the vent pipe if it only leaks when completely full. Attached is photo of the top side of a tank.


  • Bighorn Gray Tank IMG_3667.jpeg
    Bighorn Gray Tank IMG_3667.jpeg
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Thanks for the responses everyone! I have fairly easy access to the drain and vent ports in to the tank so will do a test and fill-up the tank and watch to see where the leak is coming from.