Also ... vapor pressure inside any propane tank not only depends upon temperature ... it depends upon the area of the surface of the remaining liquid in the tank.
What this means is a vertical tank of any given diameter and height and with any given amount of liquid propane in it will have less internal vapor pressure most of the time than a horizontal tank of the same given diameter and length with the same amount of liquid propane in it.
Our RV's 18 gallon propane tank is a horizontal one .... so it can probably be used way down to a fairly low level.
