Bex,
I wouldn't worry about those sensors too much. They are almost guaranteed to not work.
Even the newer trailers, which use a much more sophisticated way of sensing, are famous for sensor problems.
Two things that are guaranteed to show your tank condition: a flashlight down the toilet for the black tank, and if you see water in the shower, they grey tank is full.
You're on your own for the fresh water tank
__________________
“Courage is being scared to death, but saddling up anyway.”
...John Wayne...........................