Most likely you are actually empty and the sensors are not working. Take a five gallon pail and put it under the outfall, then let it come out. Yeah, it's sewage, but just take it and dump it down the toilet. If you only get a gallon or two, or even nothing at all, you will know the sensors are not working. Lots of ways to clean the tank, some will possibly make the sensors work for awhile.
If it's a straight drop into the tank, use your flashlight to check it out. When you find a spot to dump, fill it up and then drive around and then dump and see if that helps.
