Ok, I keep seeing that you should keep the black water valve closed when you are connected to a campground sewer. I understand that if it is left open that liquid will drain but solids won't. So how do you get the solid to go to the sewer? Do you just wait for the black tank to get to the emptying point, then open the valve and dump into the sewer? Or do you just hang onto it until you get to a dump station? TIA!