2014 37 ultimate rear gray tank venting issue

When emptying the rear gray tank, it will drain 2/3rds but at 1/3 full it slows down considerably and I hear burbling. When I shut the gray valve it has some time to catch up and will drain again until the lack of airflow slows it down yet again. I'm assuming this is a venting issue. Does anyone know where the vent is? I found one under the outsize sink but removing the vent cap did nothing to ease the suction. Any help would be appreciated.

Thanks,
Kent

Kent, if I am correct, it comes through the roof. But it depends on what drains into the "that" tank. Is it kitchen or shower and bathroon sink. The vent stack could have something in it. If you have a "snake" handy, try gently running it down through the pipe.....but you have to find out which one it is. BTW, if you are gauging how much is left by the monitor panel.....good luck with that. They are never correct. One other thing, how level is your coach. If its leans towards the door side, front high or low, it may not drain completely.

The front could be slightly downhill.... which in theory, if the vent pipe is horizontal for some length, could be full of water. That would cause the exact symptoms I'm experiencing. Thanks for the tip. I'll look into this!