[uwe]

This weekend, while working on the Airstream, I suddenly realized why my huge looking add on grey tank would "fill up" so quickly...duh!
It has 2 lines connected to it, one from the streetside coming from the galley sink, and one from aft, coming from the shower/bathroom sink. The one from aft also doubles partially as the drain, the smaller dump valve tees off from this line.
Both lines go in the bottom of the tank, the shower line a bit lower than the one entering from the side.
So I went searching for the vent connection, and lo and behold, there is none! So, once the water level goes up to the top of the inlets, it's finished! No place for the trapped air to escape. It looks like a 30 gallon tank, and all the while I was getting frustrated by how quickly it would "fill up".
So next weekend I will drop the tank and see about getting a vent tube connected to one of the roof vents. I believe this will make a huge difference in the capacity and useability of this added on grey tank.
If anyone has any suggestions on how to add a vent line to the top of the tank, which is hard plastic, then I would be greatful. ( size/dia, type of hose/ type of fitting etc)