Like lmichael, I think that lodged toilet paper might be the cause.
This time of year when the RV season for us is wrapped up, I add a 1/4 gallon of bleach into the black & gray tanks via sinks and toilet and fill with warm water to the top, then let soak for an hour or so, half the time with the tank valves open and then closed. The bleach & warm water solution loosens contaminants throughout and also destroys odors. I follow up with a full tank of fresh water because bleach will do harm if left for extended periods. I do it primarily because our PC is parked inside our garage, located under our master bedroom. I can't afford to have sewer gas. Other benefits include accurate tank sensors, clean traps, bleached white sinks & toilet. That process is work but worth it for us.