The joker valve can block back flow, but it can't CREATE back flow. Something is pushng flushes back to the toilet. A brand new joker valve can slow it down to slow seepage....if it's coming into the bowl as a flood, the joker valve is WAAAAY overdue for replacement, but replacing it won't solve the problem.
You only answered one the questions I asked you. I didn't ask the others solely out of curiosity, I need as much info as possible to diagnose your problem. So I'd appreciate the answers to the rest of 'em.
However, based on what little info you've provided, I have a strong suspicion that the culprit is a blocked holding tank vent. When air displaced by incoming flushes has nowhere to escape, the tank--the whole system--becomes pressurized, preventing flushes from even getting TO the tank. And btw, unless your holding tank is full to overflowing, it's highly unlikely that it's actually holding tank contents that are backing up, 'cuz the inlet from the toilet is at the top of the tank and contents below that fitting don't jump into it. So what you're seeing is waste in the toilet discharge line that isn't making it to the tank.
So before using the toilet again or especially before trying to pump out or dump the tank, make sure the tank vent is clear, 'cuz continuing to flush against a blocked tank vent will continue to pressurize the system...a pumpout or overboard discharge pump will pull a vacuum that won't let it pull out more than a gallon or two.
Blockages rarely occur in the vent LINE. The vent thru-hull and the other end of the vent line--the vent fitting on the tank and that end of the hose--are almost always where blockages happen.
Start with the vent thru-hull: use a screwdriver blade or whatever works to scrape it out. If there's a screen in it, knock it out...screens cause more problems than they solve. Next, open the deck pumpout fitting...VERY CAREFULLY, with a hose at the ready, 'cuz a pressurized tank can spew a geyser. Then remove the vent line from the tank and scrape out both the tank vent fitting. Now flush the toilet to see whether there's still back pressure in the system. If not, you've solved the problem. If there is, there may be a clog in the toilet discharge line.
Peggie
"If you can't explain it to a six year old, you don't completely understand it yourself." --Albert Einstein