Most farm tractor tanks had a "fill" valve and a "vapor equalizer" valve. When the vapor equalizer valve was opened and the pressure in the tank was lower than the pressure in the supply tank the gas would flow into the tractor tank. The more the equalizer valve was opened the faster the pressure would drop and the faster the tank would fill.
You are correct "vent and fill". Didn't take long!! The same process is still used today. I only assume OSHA,EPA, and other well meaning organizations would frown on the "vent and fill" procedure. In the proper hands l.p. gas is the best type of fuel an engine can burn !!
I think many farms had l.p. tanks mounted on gears that could be pulled to the field for refueling the tractors.