Couple of thoughts on measuring propane tank contents. Weight has already been mentioned, another is temperature, as propane boils off, the temperature falls, you can often see a layer of frost on a tank, where the liquid is, so you could have a temperature sensor at the minimum level. It would probably only work while drawing off the gas though.
Another possibility is using an ultrasonic transmitter and receiver. You would need to experiment, but possibilities are transmitter and receiver at the top of the tank to see if you can reliably detect the liquid/gas interface in the tank, by the pulses bouncing off it.
Or a transmitter at the top and a receiver st the bottom to measure the speed of sound in through the tank. This would rely on the speed if sound being different for the gas and liquid.
Experiments would be with a full tank, an empty one and one partially full.. (getting the level of the partially full one by weight or temperature level detection)
Or a transmitter and receiver mounted opposite each other in the side of the tank at a minimum alarm level. When the level drops below that point, the speed of sound changes, giving an alarm condition.
The experiment would be on a half full tank, measuring time above and below the gas/liquid layer. (You could use that method for calibrating the level detection using the sensor at the top and bottom of the tank suggested earlier)
(Obviously the sensor would be outside the tank, transmitting the sound through the metal, so it would need to be in good acoustic contact with the external surface)