Handling “hard dried stuff” in the black tank ?

You can, if you have time, put a quart of home septic tank treatment in the tank with a quarter fill of water. Let it sit for two weeks and drive. You could add the ice and then drive, but all that enzyme treatment should have eaten it all up. Did this to a fully clogged tank and it was so clean the OEM tank level system worked for the next five trips like it did when new.