How to manually check black/gray tank levels?

[jackconn]

I've got a 2017 Rockwood Mini Lite 2014S.
A consistent comment on this and other forums about tank level indicators is they are notoriously unreliable. So....how does one check the tank level if not sure about the 1/3-2/3-Full indication? An idea for the black water tank seems simple but a bit gnarly...with fresh water depressurized, step on the flush lever to open the flapper and use a stick of some sort to measure tank level. But there's not a direct path into the gray water tank. Ideas? Thanks!

[RandK-M]

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.

Ray