Sounds like you might be new to the world or RVs. Tank sensors are known for inaccuracies. Experience will give you an idea of how long you can go between dumping. Donít rely to much on the sensors. Gulf Stream indicates that your gray and black are both 52 gal. Not bad for black but your gray will fill pretty fast.
FYI, when I empty mine, tanks will normally show half full. A couple rinses may get them to show empty but as soon as we use them, they show half full again.