First Comment is that the sensors are not trust worthy - mine are very inaccurate and have been since new.
Try closing the grey port and stick a use a hose pipe to fill the tank as much as possible. You'll know you're nearly full when the drain in the shower stops draining. Then open the grey tank valve and full drain the tank - try to tilt the camper so gravity is helping you out.
Otherwise, you can use a "plumbers snake" to come in through the valve (careful here - don't force anything);
Gerry
Gerry