Determining the level of propane in a tank is harder than
you might think.  Propane boils at -44 degrees F; only the
pressure in the tank keeps it liquid, the same as your
sealed radiator system keeps the water liquid at temps
above 212 degrees F.

The amount of liquid propane in the tank has no connection
to the vapor pressure of the gas in the space above it--
only temperature does.  At -45, the vapor pressure is zero.
At -20 it is 11 PSIG, rising to 257 at 130 degrees.  This
is why you cannot measure the remaining liquid propane by
any kind of pressure gauge.  The pressure remains constant
(at a fixed temp) until the last molecule of liquid propane
boils.

What you can do is attempt to find the level on the tank
where the liquid/gaseous line is.  When you are using
propane, some of the liquid is turning to gas.  The
molecules becoming gas take the heat, leaving the liquid
colder.  Ergo, just find that temperature gradient point.
Again, compare to water.  A pan of boiling water stays at
212 degrees (at sea level--the pressure factor again) until
all gone.  All the heat you apply to the pan goes out in
those hot water vapor molecules.  You can't get the liquid
water hotter than its boiling point.

Campingworld (for example) has two "gauges" that do that:
http://www.campingworld.com/browse/products/index.cfm?prodID=4800&src=SRQB
or http://tinyurl.com/dv8qq
http://www.campingworld.com/browse/skus/index.cfm?skunum=4317&tab=desc
or http://tinyurl.com/drkeu
How well these would work on our skinny horizontal tanks, I don't
know.  How well they really work on larger DOT tanks, I don't
know either.  ;-)

Next time I'm near CW, I'll get them and experiment.  Of course
it may mean that I also have to get the one-piece RVer jumpsuit,
some 12V patio lights with American flag motif, the giant outdoor
carpet and the gizmos to dispense paper plates one-at-a-time....

                                Richard A Jones
                                Boulder, Colorado