I'm not so sure it was a disc but a good suggestion. We ended up using Liquid Drano (the safe version) at the end of the season and it drained okay after that. There was a "pyramid of p**p" under the toilet that would not go away. My theory is that because the tank is only about 3 inches tall below the toilet, glue hanging down an inch or more traps the TP and solids. I guess we will see what happens this time.
It's sure not fun at a CG without sewer hookups where the honey wagon only comes around once a week and your tank plugs up right after it left.