Another reason to keep valves closed....we were at a site and had the grey water valve open as my son and his fiancé were visiting and it was shower time. I forgot to close the valve and a few hours later the motorhome was filled with a sewer stench.
I closed the valve and aired the moho out. Talking to the park staff (county park) he told me that the sewer hookups were installed without a proper venting system and the sewer smell was backing up to the highest point in the campground...my sewer hookup!