If your toilet is right above the tank, a flashlight is all you need. Gray tanks are usually pretty accurate.
On a side note, I installed Hurst Miracle Probe sensors and most of the time they work fine. Sometimes immediately after dumping, the black tank doesn't read empty, but after a few minutes it clears. When I use the black tank flush, they always read perfectly after dumping.
2017 Rockwood 2109S
Me Her And a furry little dog
US Navy Veteran