how a car measures fuel in the tank
I have never done this but it may be worth following up, I 'm assuming the fuel is in the tank:
Place a small audio transducer (a small loudspeaker) at the top of the tank. Make the back of the transducer airtight and the front open to the tank through a small hole.
Now send a pulse of about 200mS into the transducer so it makes a pressure wave through the hole into the tank. About half way through the pulse, measure the current into the transducer. If my idea works, an empty tank will have more air in it so the compression from the pulse will be less and the current low. If it is full, there will be less volume of air to compress so there will be more resistance offered to the transducer and the current will be different. If the principle works, it may be possible to position the transducer further away and use a rigid pipe to conduct the pressure wave.
Just an idea...
Brian.