I had it happen to me because I bought a unit used that had never left a campground, shortly after purchase I had the pyramid.
I went to the hardware store and got a water blaster, it s a black rubber bladder that expands with water pressure and blows a clog out of a sewer line. It goes on the end of the hose and is about the size of a cucumber, I put it through the toilet just at the top of the tank and turned the hose on, after the blaster filled the gush of water broke though and got the clog out, a few more finished clearing the tank.
I always knew to keep the valves closed but the person that had the unit for the short time before I bought it from them did not get that bit of information.
