Check under sinks for vent valves. If these go bad stink from gray water is awful. Can't believe manufacturers get away with this shortcut. Easy to replace as they are screw in and available at most hardware stores. I always carry a spare.
My theory is if you open drain valves the seal is broken to allow air to vent and they don't reseal due to age or poor sealing surface the stink begins. Once I replace them the stink is gone. 10$ 2 minute fix.
2008 Cardinal 30RKLE 5th wheel sold
2006 Rockwood 2607, 2001 Traillite
55 nights 2009, 53 for 2010
44 for 2011, 38 for 2012, 35 for 2013, 51 for 2014