I've always been a leave the grey open camper and let shower, washer, sink water just flow out continually. BUT, I now think that results in a bit of scum, grease, etc attaching to the hose or staying behind in the bottom of the tank.
So now will leave the grey closed until the monitor says its time or my wife yells out "there water coming up through the shower drain". And we also pour some Dawn liquid down the sink drain every so often.
Bill and Debb---2010 CT Coachworks Siena 35V
1999 Jeep Cherokee Sport---Roadmaster Falcon 5250 -
1 Cavalon (Scooby)