fleetwood grey water drain problems

I don't understand it either, grey water tank is mty, I put a garden hose down the vent and water ran out of the gate valve drain so the vent is clear. It is plumbed the same as a house sink drain plumbed with a vent stack. The water to drain is 1/2 tub of one side of a double sink. Seems if the water will not let air go up the stack. I even instead of plugging the stopper right out, just only lifted a bit but did the same thing. Quess it is a good question for a Fleetwood factory rep.