It's amazing how much people worry about their sewer tanks, both black and grey. For many it sounds like an obsession.
Many say, just go out and operate the drains every time it's needed, but forget that not every coach or coach owner is the same. My coach has a deep FWS slide that sits very low. On newer Newmar coaches, the passenger FWS, slides way out and then drops 6", straight down. This accommodates the all tile floors. Couple that with a bad back, and multiple trips to dump tanks is not something I want to do. So just because something works for you, take a moment and realize it doesn't work for everyone.
Here's another wrinkle....many new coaches, like mine, use macerator toilets so that the manufacturer is not forced to place toilets only over tanks. On my coach, black tank fluid is about the same consistency as grey tank fluid.
Again....different coaches, different uses. You have people on here with a trailer or smaller Class A with no dishwasher or laundry. Leaving the grey water shut while using either of the above CAN be foolish, since you're not in control of how much water drains. So when you say you leave your grey closed, try thinking outside your realm about how others camp before making absolute statements.
Don & Mary
2019 Newmar Dutch Star 4018 (Freightliner)
2019 Ford Raptor