Anyway, we usually keep both valves shut, for multiple reasons.
First reason is we are hooked up all winter, and it gets cold. This winter we had a deep freeze (negatives and single digits). I am used to sub-freezing temps, but it usually doesn't get that cold. Needless to say, the drain line froze up. After several hours, I had it almost completely thawed, except for one spot...and it broke!
We now keep the valves closed, and use pvc piping during the winter.
The other reason is the gray tank makes for a good washer after draining the black. When hooked up for the summer, we tend to keep the gray open, then do a partial fill-and-dump when the black tank is ready to dump.
1992 Dutchmen 30FK
2 adults who don't know they are adults, 3 furry feline children