[pairodice]

It may be the Dupree Valve (the flapper thing inside the plunger thingy)... if you remove the top (two screws) you WILL get raw sewage everywhere!... you can unscrew the fitting (the 4" or so big screw that holds on to the coach end of the plumbing) while holding it tight against the coach end of the plumbing - it doesn't have that much pressure on it... then pull it away slowly to see if the plunger thingy is plugged. It will let you know pretty fast and you can screw it back on if needed. You might find a dumping spot or get a BIG bucket to catch the residue.