Class A Diesel-Gray Tank

Lots of ways of doing it. I have left gray open until the day before departure for years. Close the day before to accumulate shower and wash water to provide good flush behind the black to clean the hose. Also flushes the bottom of the gray tank.

I think it's best to leave it closed & dump it when it gets full. Plus when you go to flush the black tank, having a good deal of grey water to flush after the black dump helps.

Leaving it open slowly collects food ,soap etc. on the bottom of the tank,also closing it builds up a good charge of gray to flush the hoses after dumping the black.

Leaving the gray open when hooked up is fine, I do it all the time. The black on the other hand, should stay closed until you have 3/4 full and are ready to dump.