But unlike coffee, ice cream, etc., where the reduced volume or weight
is on the package, the propane tanks have no indication that the they're
not being filled as full as possible. This is simply fraud, and the
retailers and suppliers should be sued in a class action lawsuit.

They usually have a level indication on the side. The change in
temperature between empty and occupied space displays on the indicator.

Where is this level indication? I have been using propane tanks for
years and I have yet to see a new or refilled tank come with a level
indication.