I agree I would never trust a tank level indicator.
The morning before I plan to drain the black tank I fill the toilet several times and watch as I flush the toilet. If the tank is near full you will notice the toilet draining different or slow.
Then with the black valve open and the flush valve open I watch the clear adapter for material coming from the black tank.
Also while dumping the black tank I have the wife fill the toilet to maximum and flush the toilet. This will help dislodge solids in the black tank. I have her continue the process until I do not see any thing except water in the clear adapter.
