I know I have mentioned this before.  My 1983 ASI diesel camper has a
propane tank from Manchester tank with a built-in level gauge.  The gauge is
made by the J. Y. Taylor Company of Garland Texas.  I doubt that this could
be retrofitted to an existing tank, as it is installed in a welded bung on
the side of the tank.  Incidentally, the tank is mounted on the right side
below the sliding door, possibly counterbalancing some of the kitchen.
Anyone interested in purchasing a new tank may want to pursue this further.
I found this address on Yahoo.

J Y Taylor Mfg Co
101 W Avenue D
Garland, TX 75040
(972) 276-1148

I hope this helps someone.

Mike Snow
Camp Pendleton, California
1982 Westfalia 1.6TD
1983 ASI 1.6D