The vent pipe to my grey water tank was installed too low in the tank, and when the tank was about 60% filled, it was blocked from venting and smells came from the kitchen sink drains.
When I removed the thru the roof vent cap, I noticed that the vent pipe was installed about 2-3 inches below the upper surface of the roof. I also removed the under counter air-gap? vent, and plumbed it directly to the exterior with flexible CPAP tubing. I also installed a Venturi roof vent, and do not now have issues with unwanted smells.