Yes there is, but it's not added to the feed line. It's an astonishingly simple device. It's a liquid crystal thermometer mounted on a magnetic strip. You apply the strip (after it's been flattened out from the packaging) to the tank in a vertical orientation.
Pour boiling hot water (is that redundant?) over the tank area around the strip. This causes the propane in the tank to vaporize in that area, cooling the liquid down and warming the gas. The thermal capacities of the strip change color and the line between the two colors shows the level.
I've used it a few times, although my color-blindness required another set of eyes to determine the correct level.
Since then, I've purchased a Flir-One which also presents temperature differences by color change. No need for the strip, just the hot water on the tank and the level is clearly indicated on the screen.
The magnetic strip is orders of magnitude less expensive than a Flir-One, but far less entertaining.
The image above shows my tanks. The left one having been doused with hot water, the right one in progress.