RV water fills have main line and also a vent line so the water will fill faster. Without the air vent you'll get a lot of back flow when you try to fill the tank.
I was also wondering if, without that venting capability, there would also be a vacuum created inside the fresh water tank if you use an air-tight seal (that is, no vent) and you begin to pump water out of the tank...it could make the water pump work harder than it should have to. I don't know if that would actually happen...just a thought.
i just found this thread started by safaritim. scroll down and you'll see a picture of the inlet and another hole for the vent. not sure where he got the vent from, i'll shoot him a PM.
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