There are 231 cubic inches per US gallon, or 0.134 cubic feet per US gallon, and 500 gallons x 0.134 cubic feet/gallon would be 66.8 cubic feet in the tank. So if you release a pressurized gas from a 175 psi tank to 14.7 psi (sea-level), the gas will expand to 11.9 times more volume, or 786 cubic feet. Assuming there was negligible expansion under pressure, the volume of the tank remains 66.8 cubic feet (Was this a trick question?).

But if this is an Liquid Propane tank, then it's a different matter. In addition to the pressure difference, there is a phase change from liquid to gas. There are 36.41 cubic feet of propane gas in one gallon of propane liquid. That is measured with the gas at 60 degs F and 14.696 psia (from the Natural Gas Processors Handbook). That works out to about 18,200 cubic feet.

But if this is an Liquid Propane tank, then it's a different matter. In addition to the pressure difference, there is a phase change from liquid to gas. There are 36.41 cubic feet of propane gas in one gallon of propane liquid. That is measured with the gas at 60 degs F and 14.696 psia (from the Natural Gas Processors Handbook). That works out to about 18,200 cubic feet.