The bottom of your tank contains. This is the quickest and easiest way to get a feel for the propane level in your tank. How Long Does an RV Propane Tank Last (Heat, Fridge, Water All you need to do is, multiply the percentage that the gauge is showing by the total capacity of the tank. How to read your propane tank gauge . We thank you for your diligence in monitoring the propane tank level to avoid an outage from occurring. Prevent that from happening by checking your tank gauge regularly and calling your propane supplier when the level drops below 30 percent. Tanks are filled with liquid propane, and are only filled to. If the gauge reads 40 percent, your tank is half full. Additionally, when your propane tank is full, it will read 80%, not 100%. Since propane expands and contracts inside the tank as outdoor temperatures change, propane tanks are filled to about 80% capacity. The gauge usually looks a lot like the gas gauge in your car, with numbers that show t...