I leave both valves closed while hooked up at a site. when either tank gets to 1/2 to 3/4 full, I dump both. (Its almost always the gray tank that fills first)
I always dump the black first, then walk the drain hose to make sure it empty. Walking the hose - I start at the MH and simply lift the hose 6-8 inches to create a hump to drain the hose, I then lift the hose a little all the way to the sewer to make sure the hose is empty.
I dump the gray the same way, and again walk the hose to make sure the hose is empty. Dumping the gray tank last flushes out the hose..
If you leave the hose connected to the sewer, many sites require that you use those bridge things to elevate and tilt the hose. These are nice as they don't allow to much water to get trapped in the hose.
2001 Coachmen Mirada (Ford F53 6.8L V10) - Toad 2001 Civic
It won't do MACH 2, but I can get a sandwich and take a pee.