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Hi my name is Robert....
A few ideas for you..... To start what I would do is dump about 1 cup of liquid soap down the toilet and several bags of ice and then drive the unit, the more turns and bumps the better, many times the ice will push on the obstruction and scrub the tank. If that will not work I would get a back flusher that goes onto the discharge pipe, most RV stores have them, you hook it up and then hook a hose up and it puts water pressure in the area of the discharge valve. I would also get a coat hangar and run it up the discharge, mark the hangar, and then open the valve and see if the hangar goes up further into the pipe, if it does not then the valve might not be working, if that is the case you would need to have the tank pumped out and then the valve replaced..... Make sure any vents on the roof for the tank are also clear, the tank needs to vent properly to drain. Most times the blockage can occur at the valve, and need to let the tank get to 3/4 full before dumping as gravity will help do the work... IF you have more questions or concerns please feel free to contact me.... Robert