Many on here use a third gate valve at the hose connection and after empting the black tank use the gray water to back flush and drain the black tank. Then the cheaper among us, simply pick up the hose above tank level and let it equalize, then let the hose back down.
Another gate valve is on my list....
Kelly and Jerry Powell with Halo (Lethal White Aussie), Nash the Rat Terrorist, and now Reid, the "Brindle we have no idea puppy"
2020 Grand Design Solitude 390RK-R