Air in the propane line

[Icemaker]

When the motor home is brand new and the propane tank never charged.. YES air can get in the lines.

Once you have gas in the tank and blead it I keep wondering how air can re-enter the lines.. (Well as it turns out it is possible)

Here is how.. If you turn off the valve on the tank so tank pressure is not present, then via expansion and contraction of the lines due to heat they can suck in a bit of air. IN THEORY, since the regulator will bleed excess pressure.

IN practice.. I've never see it happen.  But then I only close valve when re-filling.

Yes John, when the line has been "charged" w/propane @ 11"w.c.  than the entry of any air is not possible..When tanks are exchanged and the "charge" is lost than there is the possibility f air incursion into the system. Also if there is a leak in the delivery system and the lines are allowed to DE-pressurize to atmospheric  conditions than propane being slightly heavier than air will leak out at which time your hypothesis  "Here is how.. If you turn off the valve on the tank so tank pressure is not present, then via expansion and contraction of the lines due to heat they can suck in a bit of air. IN THEORY, since the regulator will bleed excess pressure." can apply.
And again I agree w/you and applaud your post.