My sister spent a few weeks with me this past summer and wasn't using enough water with each flush. She forgot. What happened was a plug of tp right where the down pipe has a turn entering the black tank. The plug wasn't total and would allow slow flow into the black tank so with each flush more would build up, but it will seep down and wasn't noticeable by me. It finally plugged completely. I could actually see part of the plug by looking down from the toilet with a flashlight. The black tank flush wouldn't get it and even flushing with the toilet bowl completely full of water didn't break it loose. I picked up a wand from a local RV store, hooked it to the hose from outside and one trip down to the black tank solved the problem.
2006 GMC 2500HD CCSB 4x4 Duramax/Allison, Titan 52 gallon fuel tank, Prodigy Controller, B&W Companion Hitch
2010 Jayco Designer 35RLTS, Cummins/Onan RV QG 5500 EVAP, Progressive Industries EMS-PT50C, TST Systems 507 TPMS, RV Flex Armor Roof