[JRTJH]

An "alternative" to closing the gray valve and accumulating "used soapy water" to use when flushing the black tank is to close the gray valve a few minutes before dumping the black tank, filling the gray tank with clean water (actually flushing the gray tank) and when it's full, then go about dumping the black tank, flushing it, then using the "cleaner gray water" to flush the sewer hose. While it may be "somewhat wasteful" it will help remove any "sour smell from the gray tank, especially if you add some dawn to the water to help "flush and clean" the gray tank.

As for leaving the gray tank valve open while connected to campground sewer, there's no downside as long as your P traps, HEPVO traps and vents are functional. If you have a issue with any of them, you could get campground sewer gas migrating through your sewer hose, through the tank, up the drain lines and into your trailer. If that's happening, you'll know it soon enough. If you don't smell it, you don't have a problem. To "combat" that, form a P trap with your sewer hose, so a part of it is filled with gray water and that will prevent sewer gasses from migrating up the hose.