I understand the idea of closing the valve to fill up the black water tank, but just make sure to do one rule: If you have to leave, before you get up for -any- reason, open the black tank valve at the -minimum-, but even better, take time to open the tank valve, turn off the water, and disconnect the hose from the flusher.
I have seen several incidents of what happens when the dump valve is closed, but the black flush is still in use... if one is lucky and all seals are good to go, and able to stand eleven feet of head pressure, the water will head out the tank vents on the roof. If the toilet seat's seal is any way faulty, the water will drain from that into the rig.