Handling “hard dried stuff” in the black tank ?

I guess you learned your lesson the Hard way:) You either are full time and leave the black and gray tank valves open or when camped you also left the valves open. NEVER EVER DO THAT. Look up Poop Pyramid in google. I would shut the tank valve boil as much water as possible and pour that hot water with the rid x. Try to get at least 20 gallons of hot water and let sit for a few days. Hopefully that will break up the poop and get it into suspension to then drain out and flush. Doug