Something is not right with your system. A propane system that is shut off should not leak propane to the point that there is air in the lines. Sure a lessening of the pressure is one thing but getting air in the lines is completely different. Even when I had tanks to switch the lines would get air in when switching a tank but they would not drain completely. Do you have lots of propane in the tank?
First check to make sure that all the valves are off at the stove/oven/ hot water if you have a pilot light, sometimes something gets left on or a pilot/burner gets a leak and does not close 100%.
I would have a soap test done. Take it to someone to have it checked who is trained in this.