Andrew Sarangan wrote:
The reason for the weight sensor was because he did not want to
penetrate the fuel tank to add his sensors.

An ultrasonic transducer mounted outside a tank might work - it would get
one reflection off the surface of the liquid in addition to reflections off
the tank walls. The tank wall reflections would remain constant and could
be ignored (or even used for calibration) but the reflection off the liquid
surface should be detected and time-of-flight or sonic interferometry could
be used to compute its location.