The sinks and shower have p-traps built into them, which blocks the sewer smell. When it's just the wife and me, I keep the bathroom gray tank valve closed. When family and friends visit, due to the larger number of showers, I keep the valve open. But, on the day prior to leaving, I close the gray valve to ensure I have some 'clean' water to flush the drain hose after draining the black tank. I also keep the kitchen sink gray valve open all the time 'cause it's a pain to climb under the slide to open and close it.
Recently we had a sewer smell that turned out to be a leaking propane regulator valve. Other than that, in 4 years, we've had no sewer smells.
2014 Voltage 3600
2013 Chevy 3500 CC DRW
2011 Harley Ultra Limited 2016 RZR 900