If you want to get your black water tank clean, the easiest and fastest way, here is what you do.
Buy three bags of ice cubes. Fill the tank half full of water and dump the ice cubes down the toilet into the tank.
Drive your rig until the ice cubes melt. When you dump the tank the ice cubes will have scoured all the hard deposits from the tank and they will flush out down the drain.
If you have a vent problem and the pipe has not slipped down, this might help.