Black Tank clog

When you open the flapper you can see water sitting in the pipe just below the flapper? In that case sounds like you have the pyramid of death. A big pile of fecal material and toilet paper right where the pipe dumps into the tank. You need to fill your tank and put some rid x or something along those lines. Let it sit and eat the big pile of poo. I am sure somebody else will pipe in with the best stuff to use. If it was me and I could get a garden hose in there I would try to water blast it away. Hopefully you can hook you stinky slinky to a drain and let her rip. But more than likely you need to let it soak and dissolve. You get that pyramid when you leave the blank tank valve open and use the pot and the water leaves the tank and leaves the solids behind. Thats why you always want to let you blank tank fill and dump it when it is nearly full so all the solids wash down the drain. After dumping your tank,you should put several gallons of water in the black tank. Don't ever leave it dry when in use.