It unfortunately is all about vapor pressure.  The vapor pressure of lpg is
constant at a given temperature.  No matter how much liquid PG is in the
tank .  One ounce or three pounds.  Don't blame me it's the law.  The
Universal Gas Law and how propane in particular in two forms liquid and gas
coexist in a closed container.  FWIW at 60F you'll have about 92PSIG (pounds
per square inch, guage) regardless of how much propane is in YOUR tank.
Provided it has headspace or 20% of tank volume.  So until the volume of the
liquid phase reaches zero the guage won't change much for a given
temperature of the tank.

The guage will read the pressure in the tank  but it will not tell you how
much gas remains in the tank until there is no liquid propane left and then
you have three gallons of propane gas at 92PSIG.

Fizzix, just fizzix it's always just fizzix .. apologies to the estate of
Richard Feynmann

http://www.propanecarbs.com/propane.html