Success!
It took a while. Had to keep emptying the tank and then using the spray wand through the toilet opening. Then empty the tank again, and repeat with the wand. Finally, the plugged area broke free and the tank emptied all the way down. There is now a clean tank bottom, where previously always had a puddle of water. I guess it was never completely empty since I owned it.
The sensor gauge now reads true empty. So the gauge readings were probably always right when I thought they were wrong.
I'll definitely use a clear flush device. I was
going to buy this a while ago and never got around to it. Now I'll be putting my order in. Should be a Hek of a lot easier than the wand.
Thanks everyone for your help.