For the last three years I've owned a 2001 Bounder 36S pusher and it has been a wonderful first RV for my wife and me.  However, I've been chasing an odor problem from the rear of the coach that I just can't locate to repair.  Now, I realize there will always be some odors in an RV but this odor is very strong at times.  I'm confident it's from the gray tank and also quite sure it's coming from behind the washer combo.  To summarize,  water stays in the trap, I've replaced all vent valves in the coach with the OE, then an upgraded valve.  I've inspected the vent stack and it's all clear and it has a cap/cover.  The drain hose isn't restricted and I smell nothing from inside the washer.  We discontinued the use of the washer and I capped off the drain pipe and no longer smelled the odor.  Finally, I had both holding tanks professionally cleaned.   When I started using the washer again the smell returned.  I'm at a loss!  I've purchased a newer motorhome and don't want to sell this one with the odor but don't know what else to do.  Any ideas?  Thanks in advance for your thoughts.