I understand your problem. A lot of attention is paid to cleaning a Black-Water Tank, but people usually ignore the Gray Water Tank. Gray tanks do not have built-in showers and when something does get hung up on the sensors it can be a problem.
If you can imagine a piece of food hanging there on the sensor, you can understand your problem a little better. Eventually, you can hope for that piece of food to rot and fall off of the sensor, but too often the piece of food dries up and turns into something resembling a piece of jerky. When this happens it has shaped itself to hang there, on the sensor and considering it is high enough to be on the 2/3 sensor, it probably doesn't get wet too often so that it can soften and fall off.
Considering all of this useless information, there is one trick you can try.
Fill your Gray tank up to where you estimate the sensor would be, let the water sit for a day or so, and then drive the Rv up and down the road to hopefully allow the sloshing water knock the piece of food off of the sensor.
This isn't a new trick. In fact, a lot of people have this problem with their Black Water tank and they do the same thing but they will add a couple of bags of ice cubes down the toilet and hope the ice knocks the toilet paper off of the sensor when the built-in shower doesn't work.
PS. Keep that trap in your sink. You will be surprised what gets caught before it goes down the drain.