Tex
They measure the propane by a gage on the truck as it comes from the truck to the tank, so it is highly unlikely that anyone is screwing you. First idea, go to the propane company and look at their tanks they have in the yard waiting to be sold or rented, pick out one the same size as yours and check the tag. Check with your neighbors, one of them likely has the same size. Around here most tanks are either 250, 500 or 1000 gallons, not hard to tell the difference. How to convert percent on the gage to gallons requires that you know the tank capacity . For instance if you have a five hundred gallon tank, then every ten percent on the gage is fifty gallons, 1000 gal tank then every ten percent is 100 gallons. 250 gallon equals 25 gallons for each ten percent, On the 500 gallon tank if you have 40 % you have 50 x 4 or 200 gallons if the gage is accurate.. Plain ole simple basic math.

Propane Guy
Keep in mind that float gauges can be wildly inaccurate. They are basically a float on a lever with a gear that rotates a small magnet in the head of the gauge. A corresponding magnet in the plastic dial rotates with it as the level raises or drops. But those gauges don't go to the very bottom of the tank so the dial could read zero and you still have several gallons of fuel left. When properly filled the gauge should read 80%, but I've seen tanks that are full at 65% or the dial goes past 90% before the fixed liquid level valve indicates the tank is full. The float gauge is only there to give you a rough approximation of what is in the tank, it is not precise. You could pump 20 gallons into a 1000 gallon tank without the needle even moving. The meters on delivery trucks are temperature compensated and very accurate. Modern equipment also has interlock devices to prevent a driver from "riding a ticket"...In other words filling their buddies tank, leaving the ticket in the meter and driving to your house where they put a bit of fuel in your tank and you pay for it all. If you give us the outside dimensions of the tank I can tell you what size it is.

Oxoboxo
Propane Guy is once again right on the money with his answer.