Clogged Black Tank

As others have noted, make sure your valve is opening all the way. If it is, there are a couple of other possibilities. First (most likely) you have a big wad of TP sitting at the exit pipe in your black tank. Solution for this is to keep filling your black tank with water until it pushes the wad out. Second, you might have a tank cutout that floated to the exit pipe. For that you'll need to remove the valve, then remove the cutout with a pair of long nose pliers (messy job). Third, if you really do have a poop pyramid you could try a chemical like Tank Techs RX or a drain bladder like this (pushed down through the toilet): Drain Bladder