Originally Posted by wiscampsin
The vent pipe might have slid down and is sitting on/near the bottom of the black tank. Happened to us. With the cap off try lifting the pipe up and see if that helps.
Had both vent caps off yesterday. Ran water down both. One seems to go to one grey tank, the other vent seems to go to the other grey tank. Both vents take water without it backing up onto the vent pipe. I would think if one didn't down water would back up in the pipe.