Yes the porpane stays a liquid under pressure, which is why if you just hook a gas pressure gauge to the tank, it will vary with temp, but not with volume left in the tank.  

I ran into a similar issue at home with a CO2 tank on my kegerator.  The gauge never changes until the tank is almost empty for the same reson so it's impossible to know how much is left in the tank without picking it up.  I was searching for a scale that I could read over RS-232, but couldn't find anything I was willing to pay for.  They were all around $400 which didn't seem worth it.