The two most common problems that occur in RV toilets arise due to problems in the seals at the bottom of the toilet bowl and the seals of the water valves. In order to repair these problems, turn the water off, flush the toilet to clear any remaining water and water pressure and remove the toilet to make the repairs.
Standard RV toilets keep about an inch of water above the valve at the bottom of the toilet bowl between flushing to help block odors from the tank below. These seals and all seals in an RV toilet are made of rubber, which degrades over time. The seals become hardened or cracked, which keeps them from sealing properly. This prevents the normal inch of water at the bottom of the toilet bowl from being retained. Once the toilet is removed, these seals, known as blade seals, must be replaced. Put a quart of water in the toilet bowl to check the seal before reinstalling the toilet.
If the problem is that the water does not shut off properly, the broken seal is at the back of the toilet. Replace the sealing rubber, and then reattach the water line and pressurize it to make sure it holds before reinstalling the toilet.