Hi dstearno,
As Jim mentioned, you very likely have a large pile of solids that have built up. If your black tank gate valve has been leaking, even a slow leak, all the water you flush leaks out, leaving a pile of solids. Sometimes the valve can get that way if you inadvertently close it while paper is in the valve. As you push the handle, the blade compresses the paper into the blade receptacle leaving an imperfect fit.
Many of us have added clear adapters to the sewer outlet so we can see what's coming out. After you see clear water for a while, you'll have more confidence that there's nothing left in the tank or in the valve opening.
At this point you may have to carefully backflush the black tank from the sewer outlet. I say carefully because it's possible to overfill the tank causing potential messes inside, in the underbelly, and even on the roof. When backflushing, I time it with my phone. In most places, you're not going to get more than 5 gallons/minute, so 6 minutes is probably safe.
I'm using a
twist-on clear adapter with backflush and external gate valve. You may find one at a local dealer parts department. Close the external gate valve and attach a dedicated black tank water hose (not your regular water hose) to the backflush fitting. Run for 5 or 6 minutes and then open the external gate valve. You may have to repeat a number of times to break up the pile in the tank.
When you think you have the problem solved, close your regular black tank gate valve. Then also close the external gate valve. Put about 5 gallons of water into the black tank through the toilet, or black tank flush (if you have one). Wait an hour to see if you have water leaking out of the black tank. If you do, the clear adapter will be full of water.