Replace the joker valve #C254, and also double-check that the vent line on your holding tank isn't plugged up. If the vent line is plugged with sewage or paper and the tank can't vent, then pressure will build up in the tank as it fills. This pressure will relieve itself in the direction of least resistance - back into the toilet, through the joker valve. The vent line can be clogged at the though hull where it exits the boat, at the other end where it connects to the tank, or anywhere in-between.
Simple way to verify whether or not the holding tank's vent line is plugged - remove the cap from the deck fitting used to pump out the holding tank, and leave it off. Then use the toilet normally. If the backflow problem goes away while the deck cap is removed, then the vent line is clogged somewhere.