Don't run out of propane.....
It isn't so much as 'air' in system.
It is more about establishing flow and placing a demand on regulator.
The system when fully pressurized is only 0.4 psi (11" WC)
With no demand the regulator is basically closed. Turn on an appliance and depending on it's demand will determine how quickly flow can be established based on that demand (fridge 2000btu---stove top 6000/9000btu per burner----water heater 10,000btu----furnace 35,000btu)
Such is the life of a small lp system and the way RV lp system is used.