How to tell amount of propane in tanks ?

[elliott-maine]

Dometic makes an LP GasChecker that works very well. As you move it down the tank, you push it in to activate. Red no gas, green gas. The level is repeatable and accurate, and it is very reliable.

[RayJack]

AP Products 024-1001 Tank Check Propane Level Indicator worked for me but battery life wasn't much more than a year.