There are two main reasons why your RV toilet is not flushing. First, water isn't entering the bowl when the foot pedal is depressed, so waste is not washed away. Second, the water enters the bowl but does not exit. Both issues are symptomatic of larger problems and rectifying them can be time consuming and unpleasant.
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How RV Toilets Work
RV toilets work differently from household toilets; they do not use a cistern to store a reservoir of water that enters the bowl under its own weight. RV toilets use pressurized water — either pressurized by the utility supplier through a connection hose or by a 12-volt pump — that is injected around the top of the bowl, usually through several ports, which swirls around as gravity pulls it downward. This injection is controlled by a device called a vacuum breaker.
Check for Water Pressure
If water is not entering the bowl, check that pressurized water exists at the upstream side of the vacuum breaker by loosening the nut that attaches the flexible inlet hose to it. Use a crescent wrench and turn the nut slightly counterclockwise.
If water starts to bead around the thread, then the pressure is fine and the vacuum breaker must be replaced. If the hose can be removed with no water pouring out, then there is a blockage or turned-off stop tap somewhere in the plumbing system.
RV Toilet Not Flushing Water
Confirm that the black water tank is not full. Sensors and gauges fail, so it may be that the RV's central instrument cluster is suggesting that the tank is not full even when it is. Observe all the usual safety rules for emptying the tank (do so at an approved dump station, wear protective gloves and connect the sewer hose securely) before opening the dump valve. If nothing emerges, close the valve, remove the sewer hose and open the valve again extremely slowly.
If the valve opens and a blockage can be seen in the pipe or at the top of the pipe where it is attached to the black water tank, the blockage must be removed. A blocked pipe or tank can be cleared with chemicals sold under brand names like Porta Pak, Happy Campers or Camco TST.
Empty the chemical into the tank and let it work for 48 hours. Drive the RV if possible to wash the tank contents around. Then empty the tank and flush it until the water coming out is pure. Do not use household bleach; it can corrode seals and damage in-tank sensors.
Stuck or Broken Slide Valve
If the tank proves to be empty but water is not leaving the bowl, next determine if the slide valve at the base of the bowl is not functioning. It may have seized closed or the actuating mechanism may be broken. Start by turning the water supply off by disconnecting the connection hose and by checking the 12-volt pump is off. Then step on the flush pedal.
If the pedal resists movement, the slide valve is seized in its closed position. Purchase a holding tank cleaner like Aqua-Kem, empty it into the toilet bowl and follow the instructions on the packaging. If the pedal moves freely but the slide valve remains closed, the actuating mechanism is broken. It is necessary to remove the toilet to replace the broken mechanism.
If the slide valve opens and the black water tank is empty but waste in the toilet bowl does not flush away, then there is a blockage in the pipe beneath the toilet that leads into the black water tank. Wedge the flush pedal down so that the slide valve is held open, then use a straight rod to loosen the blockage. Follow this with a bucket of water to flush the loosened blockage away.