Ideally a RV propane system should not be able to get air inside the lines as it is a sealed pressurized system. If you completely run out of propane you could end up with the problem you are describing. That being said; as propane systems get older there are certain other conditions that can develop that will also cause the problem you are experiencing.
In your case the first thing I would do is have the propane system on your RV tested by a Certified RV Technician. This test should include testing your RV's Propane Regulator for proper operation as well as a drop pressure test that will determine if you have any leaks in the propane system.
The most likely cause of a leak could be an older rubber propane hose that has developed a tiny leak or a propane connection that has loosened up over time. In a lot of cases these very tiny slow leaks will not be picked up by an RV Propane Detector, but during long storage with the main propane tanks turned off they will leak out enough propane to cause the problem you are experiencing.
You might want to take a look at the video below from Pedata RV Center to learn a little bit more about maintaining the propane system on your RV.
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