On our trailer the only lights that indicate any kind of level are the fresh tank ones. The grey and black are not reliable as they get covered in crud. At their best the lights are only as reliable as your gut tells you as they are likely drilled into the tank in a very lackadaisical fashion during assembly so that a full reading is never full and an empty reading is never empty.
If water backs up into the shower pan, it's time to dump. If you can see water when you flush the toilet, it's time to dump. We can fill both tanks at the same rate by washing dishes in a dish pan and dumping the grey water down the toilet. That way they can always be dumped at the same time and the extra relatively clean grey water really helps flush out the black.
There are 2 systems that are "better" than the cheap OEM. Google "horst probes" and "seelevel" to find links to the manufacturers and several retailers as well as some reviews. Frankly, my dishpan was a lot less money and a lot less aggravation to install.
2019 F150 3.5L Max Tow
2014 Arctic Fox 22G
2005 Sunline T-2499