That could be very dangerous, ha. That may be your only alternative but IMO it should be your last for obvious reasons. I take it you looked in the toilet and verified the tank is really full? If so, it can only be one of two things, the valve stem has come loose from the gate and it is closed. I never heard of this. It would take a h_ll of a plug to hold back a full tank of fluid. But possible, especially if the wrong kind of toilet paper was used, I guess. you could fill the gray water and with the cap on open both and maybe back flush gray water into the black water. I'd be reluctant to do this with a hose as you could over pressure the line and blow it up if the valve is closed Stay with gravity, IMO.
Rockwood 2104S, 2014 Ram 2500 Diesel.
USMC 68 -70