Updated- end of thread,something in the black tank.

I would fill the tank and see if it comes out the drain. Next attempt would be with a metal coat-hanger with a nice hook on the end. Maybe a couple tries including a few fill and drain cycles with RV tilted pretty good toward the exit. Last resort would be to rent a plumbers camera to see where it was and again hook it with the coat-hanger.

Best of luck.