It is from the internal display. But the readings seem relatively accurate, since it shows as 40% when level, and as we raise the front of the trailer, a lot of gray water drains out and it reaches zero on the display after it finishes.
Since we started using our 2024 Oliver Legacy Elite II, the gray and black tanks won't drain fully unless we lift the front as high as it will go. If the camper is fully level, the gray only drains to about 40%. Anyone else have this issue?