Sure that's easy. All you need is an HX711 sensor load cell amplifier taped to the floor of the chamber where it sits under an edge of your bottle.
Full, 5 gal (US) jug or water weights 40lbs nominal. If you have the same bottom feed dispenser I have, it starts drawing air with about 16-20oz of water left in the bottom.
You'd want to calibrate first for the sensor without weight (tare), and then for a full bottle's weight (40lb) accounting for cosine error of the bottle being slightly tilted by the thickness of the sensor. Or, tape a couple of shims at 120° offset from the sensor to level out the bottle.
Finally, note the weight of an "empty" bottle with that 18ish oz of water in it, which will probably be close to the zero value.
Now you can calculate % full by looking at the ratio of the current weight between your full/empty value as:
%Water = [ (W_full - (W-18oz -W_tare) ) - W_now ] / (W_full - (W_18oz - W_tare) ) x 100