3 inch grey water drain?

Thanks for all the answers so far. When draining the grey tank, it comes out at a VERY slow rate. We have a clear elbow,so we can see the the rate of flow. We are not trying to get the last drop out of it,its slow from the start. I thought they must have left a cut out disc in it also,but the tank itself is molded as is the outlet on the tank ,so nothing to cut out. Its raining here today so I'll wait until tomorrow to keep looking. So far the drain pipe from the end where the drain hose connects is wide open as is the slide valve. I'll next try to run water down the roof vent and see what happens as long as everything is apart. Again Thanks for all the help on this