I keep my black valve closed and thus always have some water in the black holding tank. About every 4 or 5 days I open the black valve, dumping the tank. Then I hook up a seperate hose I use for black tank only and do a flush for about 5 minutes. Then I close the black valve again. These tanks are not designed to be drained all the time. If you do this you will get a poop pyramid at the bottom of your inlet from the toilet!! By keeping some water in the black tank all the time this will not happen.
If you should close your black empty valve and leave the flush hose running, it will fill the tank and come up through your toilet and flood your trailer!! Be careful