TOWB group member Faith Senie explains it this way:
“The One Place is actually pretty accurate — once you understand what it’s saying.
It reads 3/4 when you’re somewhere between 3/4 and full;
It reads 1/2 anywhere from 1/2 to 3/4;
It reads 1/4 anywhere between 1/4 and 1/2; and
It reads E anywhere between empty and 1/4.”
In other words, as soon as you use some of the item in the tank it shows the next level down.
The Water, Gray, and Black tanks read F, 2/3, 1/3, and E but the concept is the same.
Of course, for LP you always have the option of going under the van and looking at the reading on the LP gauge on the back of the LP tank itself.