I am thinking you have a real "sticky" problem. I am guessing your valve body is packed with poo. There is not enough water pressure to push through it.
To fix this problem:
1) STOP USING IT. This will only make the problem worse.
2) If the black water is already backing up into the bowl, the tank is already chock full you are kinda hosed. There is no way to add RV-Digester to your black tank.
3) Call a RV plumber and get ready for a big bill. RV tank guys hate/love this scenario. They hate it for the mess, but love it for the big bucks.
Now if calling out the troops is out of the question;
Open all windows and clothes pin your nose.
Wear clothes you intend to burn.
Wear rubber gloves and goggles.
Turn off water to the toilette so you can open the bottom valve without adding additional water to the tank.
Open the black dump valve to the sewer (use a clear elbow if you can and have someone watch for "movement"
Take a flexible water hose about 6 foot long and start pushing it through the toilette and pull it up and repeat while twisting it around (you are trying to find the open valve that is blocked with solids).
Hopefully the blockage will be opened up enough to let some of the water out of the tank.
Once the water starts to leave (and hopefully some solids too) close the valve. Add a FULL quart of RV-Digester or similar biological sewage digestion treatment and let stand for 24 hours. (Says 12 on the bottle but more is better).
Open the black dump valve and lots more should come out.
Dump till empty.
Close valve and fill black tank with water. Add ANOTHER quart of RV-Digester and let stand 24 hours.
Dump again and flush well with Turbo wand or Turboflush until water is clear running.
Throw away (or burn) clothes, hose, gloves, and goggles.
Follow correct dumping instructions in the future.