In a bit of a predicament (Black Water tank full and frozen)

Did you visually verify that there's ice in the tank? Can you look down the toilet and see the tank or is there a bend in the pipe?

I'm with valhalla. I think it's clogged from misuse.

That reverse flush connector just shoots water up the outlet pipe. If there are any bends in that, you're not getting to the tank proper. Try a flexible wand down the toilet.

Don't go strictly by the sensors.