JOKER VALVE
WHEN I HAD THE SAME PROBLEM IT TURNED OUT TO BE THE "JOKER VALVE" WHICH SEEMS TO BE A FANCY WORD FOR MARINE HEAD CHECK VALVE. IT ALLOWS STUFF TO ONLY FLOW ONE WAY. LUCKILY NOT A HARD THING TO REPLACE. IT IS WHERE THE DISCHARGE HOSE LEADING TO THE HOLDING TANK CONNECTS TO THE TOILET. EVEN THE FIRST TIME IT TOOK LESS THAN 5 MINUTES TO REPLACE. IF YOUR PROBLEM IS THE SAME AS MINE, REPLACE THIS JOKER VALVE ASAP. IF IT GETS ANY WORSE THE TOILET WILL OVER FLOW. IN THE MEAN TIME YOU MAY WANT TO KEEP THE HOLDING TANK PUMPED OUT. GOOD LUCK!