Black tank says full but it’s empty

I recently had the same issue. My black water tank is 33 gallons. I fixed it by pouring 25 gallons of water in the tank (5 x 5 gallon buckets) and tossed a enzyme packet in there and let it sit overnight. Then drove about 20 minutes to the nearest dump station, dumped the tank, then poured two more buckets of water through the toilet fast while the tank drain is open. This cleared up the sensor and also cleared whatever else is in the tank. Now I can shin a light down the toilet and see the plastic of the bottom of the tank, and no smell.

Link to the enzyme packet: https://www.amazon.com/gp/product/B00157TGXY/ref=oh_aui_detailpage_o04_s00?ie=UTF8&psc=1