All of my tanks are visible somewhere.
After calibrating my fresh water tank (knowing how much my supply hose would provide in a minute) and marking the side of said tank with a sharpie, I was able to calibrate the gray in a similar manner (5 gallon loss in the fresh = 5 gal in gray with shower running.)
I did re label the electronic gauge to show the correct quantities (mine work most of the time), but if in doubt, a quick check of the appropriate compartment is easy.
The black tank, well it's easy. Just a long flush with a flashlight in hand will give you a good clue about remaining capacity. It's never a problem as Fleetwood went 50/50 on the gray/black on mine. Wish it was 70/30.