Propane tank level monitor

I use a pressure gauge inline on the hose, after the regulator. Pressure will drop slightly as the tank empties.

If you keep your tanks (dont swap them out for full tanks at gas station), then the monitor that tapes to the side will show the gas level. you pour warm water on the indicator and it shows where the top of the liquid propane gas is in the tank.
Mine get swapped out when it is not convenient to fill them so I don't bother with this.