If you have water that won't drain in the shower, remove the vacuum breaker. If you get a puff of air out of the pipe and the water goes down the vacuum breaker is sticking. You may have more then one. The actual name of it is air admittance valve, common name vacuum breaker. You can buy them at Lowes and Home Depot.
Amazon.com: Air Admittance Valve
Chuck & Teri, 1999 35 ft. Dolphin 5350, F53 Chassis with tag axle, Chassis build date 1/99 in Mexico, Banks System, Trans Command, Air Lift Air Bags on rear, Koni Shocks, Blue OX TruCenter, TigerTrak track bars front and rear, Roadmaster 1-3/4" rear auxiliary sway bar, 2013 VW Passat TDI SEL Premium, 1994 36 ft. Avion 5th Wheel. FMCA #F430129