This seems to be a common problem. Mine says 1/3 full after I dump it. I have a farm in the middle of nowhere and fertilize when/where I want but it is annoying.
I ran a hose and filled it, flushed it. Same old "crap." 1/3 full.
Use a flash light to check it.
Last time, used a coat hanger and run it up from the outside into the toilet. Nothing much came out. Not for the faint of heart unless you grew up like me, cleaning out the cow barn as part of my chores.
Some one said that toilet paper/"stuff" gets stuck on the sensors and create a false read.
Not the worst prbblem you can have, just annoying.
They should have a flush counter, letting you know when to "dump."