[smily]

I concur that you may need vent pipe but air will always find its way to the top even if it has to "burp" back into sink or shower.

My soveriegn with 35 gal grey tank will fill up very quickly if you are not turning the water off while you lather. (Military bath).

If my wife and I take a typical shower and not turn off the water during lather the grey tank will fill up after about three or four showers. The water will back up into shower.

So I do not know if I would go through the rigomors (is that a word?) of cutting a vent pipe into grey tank.

good luck with your challenges,
Smily