I just posted a reply to a black tank sensor issue here
. Once unclogged it may help you stay that way.
Until then, if you can add plenty of digester, about 5 gallons of hot water and let it sit for a few hours. Then hook up, take the PP for good long ride and then dump. The hot water/digester mixture will help break down the clog. The long ride will help mix things up down there. If it stops dumping then shut the valve, use the black tank flush to fill the tank with plenty of water to help push the clog through. Good luck!