Propane cylinders (they are not tanks) are always treated as full per "transportation of dangerous goods" regulations because you have no way of knowing how much product remains in there. The cylinder will not be totally empty because the appliances 100% safety valve will stop the flow of propane before the last bit of liquid burns off.
Also the pressure in a propane vessel is independant of volume. It is determined solely by the temperature of the liquid inside. Soooo, a 20lb cylinder, a 5000 gallon bulk tanker truck or a 30,000 gallon storage tank all have the same pressure inside if the temperature is the same.
The other reason we don't bring cylinders indoors, especially in colder times of the year is because the volume of the propane expands as temperature goes up. This is why we only fill cylinders to 42% of the water capacity which is roughly 80% full. It is a safety "buffer" to allow the liquid to grow in volume without setting off the pressure relief valve. However. That filling procedure only allows for about 40 degrees of "swing". So a cylinder filled at -20°F is unsafe if the temperature reaches +20°F because the cylinder is now 100% full of liquid and the relief valve will vent if the temperature goes any higher.
I hope this helps!
24 years experience in the propane business and a licenced gas fitter