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I have no personal or professional experience here, but i do have a suggestion. Close the black tank valve, then add warm water to your tank. You’re not trying to melt the tank, just soften the poop. Let it marinade – hours not minutes. Dump and repeat. Edit, I have also found through personal experience that if you have a straight shot into the tank pouring a 5 gallon bucket of water directly down the hole as fast as reasonably possible causes the water to scrub the bottom as it gets pushed out across the tank. Dont recommended it if there’s poop up to the hole though