If grey water is left in the tank during hot weather too long it starts to stink fairly quickly. You can added stuff to the tank, but that is money. Wife is careful about what goes down the sink drain, and we leave the grey water line open most of the time. We also use blocks to make a dip in the waste water hose line, sort of a P trap, to keep sewer gas from coming up the empty hose and into the tank and so forth. The day before we dump the black water tank we close the grey water drain and use the stored grey water to rinse the hose after the black water has gone down. Done it since the 70s and seems to work.
2016 GMC Sierra Crew Cab 2500 HD 4x4
6.6L Duramax + Allison, 3.73 axles