Pastordug,
The tank indicators on these coaches are notoriously inaccurate and unreliable. On my coach the grey water works fairly well, but I don't trust the top (red) led lamp -- and usually drain at 3/4 full. My black indicators almost always show full in error, and like many full time occupants I simply ignore them and drain the tank once a week.
There are after market tank level meters that are far more reliable and have improved accuracy -- SeeLevel is widely discussed on these forums and is highly rated...just a bit costly in my way of thinking. I have yet to add one to my coach as my routing is predictable enough that I can drain by schedule. If the coach were used less frequently, perhaps accurate tank levels would become more important to me.
Here is a link:
https://www.rvupgradestore.com/RV-Ta...stem-s/164.htm
Chuck