We have a 2018 Jayco Greyhawk 29mv which is new to us and we are having a problem with the shower draining. Have done all of the usual things, drains not plugged, replaced air admittance valve, RV is level, grey water tank is not full. Water is very very slow to drain unless grey water dump valve is open. Called Jayco to ask where the grey water tank vent is and was told there isn't one. Any help will be greatly appreciated.