Aqua Magic V Problem

Dreaded Poo Pyramid? Not a clean job to get the pyramid broken up so it will run on out. When I first got my unit, the previous owner must have left the black tank open and liquids drained and solids stayed. I had a mess for about 3 or 4 dumps, but finally got it working. Also, I have heard if the vent pipe is too far down into the tank or it is blocked from top, this can happen. But, since toilet installed 2015, seems this would have happened before if vent pipe problem?

I turned off water outside so toilet would not fill with "more" water and make a bigger mess and then stepped on slide valve and used actually a piece of pvc pipe to break up pyramid... patience and old clothes.