Something--not the joker valve unless it's encrusted with sea water minerals-- is creating backpressure that's not allowing the flush. The most likely culprit is a blocked holding tank vent.
When air displaced by incoming waste cannot escape out the vent the tank becomes pressurized, creating increasing back pressure that prevents the toilet from flushing.
Do NOT use the toilet again or try to pump out or dump the tank until you've cleared the vent. Without a source of air to replace contents as they're being sucked out of the tank, the pump will pull a vacuum that'll prevent it from pulling out more than a gallon or two. A particularly strong pumpout can even implode a tank.
The two most common locations for a vent blockage are the vent thru-hull and the other end of the vent line--that end of the hose and the vent fitting on the tank. Start by cleaning out the thru-hull...use a screwdriver blade, ice pick--whatever works. If that doesn't result in a spew out the vent, you'll need to relieve the pressure before removing the vent line from the tank to clean them out...so open the deck pumpout fitting VERY CAREFULLY with a hose at the ready. Be sure you're UPwind of it! Scrape out that end of the vent line and the vent fitting on the tank...reconnect the vent line. If there's a filter in the vent line, get rid of it...filters actually help to create the very problem they're sold to solve, plus they're toast if they get wet--which can happen if tank overflows into the vent line..the charcoal swells and becomes a vent blockage.
To prevent vent line blockages, replace the "vent" thru-hull with an open bulkhead (aka "mushroom") fitting that'll let you put a hose nozzle against it to backflush the vent line every time you pumpout, dump or wash the wash the boat. Anything trying to set up housekeeping in it will just get a water slide ride into the tank...another good reason to get rid of the filter.
--Peggie
