Handling “hard dried stuff” in the black tank ?

Buy some ice, like at least 30 lbs worth. Dump it down the toilet into the black tank. Add a little water. Also pour down the hole a lot of liquid laundry detergent. Then, go for a spirited, bumpy and curvy road drive. Afterwards. let it sit for an hour or two, then dump. That should get all the petrified poop out.