As mentioned, the black tank stays closed until ready to drain. The gray is optional. We have not only a clothes washer, we also have a dish washer. So, if we use both, plus the shower, kitchen sink and both vanity sinks....we can fill the gray quite quickly. On those days (washing day) we leave the gray open but provide a dip in the stinky slinky to keep odors (and I've heard bugs and flies) from coming back up the sewer line and into the gray tank.
Our previous motor home only had a combo washer/dryer and no dish washer...so it wasn't a problem and would typically fill both tank pretty much at the same time. Hence, we always kept both tanks closed until 2/3 to 3/4 full.
Ron