Heating oil, do you ever check the gauge on the tank?

My sis visited my family home over the winter and they heat with an oil furnace. She checked the gauge on the fuel tank in the basement, it was a quarter full.

The heating oil company delivered and pumped a full tank of heating oil. When the delivery guy came for a signature my sis mentioned that they couldn't have pumped a full tank, as the tank was a quarter full. Oil company guy said the gauge must be faulty, so he replaced it. It read full, same as the replaced gauge. She argued, he had delivered a full tank of so many liters he said.

She looked at all the previous bills that my parents had kept in a file, the heating oil company always pumped a full tank of heating oil into the basement tank and charged for a full tank. My parents never thought to read the gauge, they trusted the company. Who has been ripping them off for decades. Nice little scam they have going, and they are only company in that area that sells and delivers heating oil. They must have been doing this to pretty much every one in that town who heats with oil.

Not exactly sure of how this all went, getting this all second-hand. The delivery bills for oil were all hand-written and the writing is quite poor, sometimes indecipherable, the oil bills that were mailed to the home list the amount of gallons/liters and the cost. Matched with the delivery orders, the bills were always slightly different, don't think anyone ever tried to match them up, they simply paid the bills.
There is talk of some legal action, although there probably is not enough accurate information for a case. The company that sold the oil was local and the owner passed away quite recently, the company that bought out the local company seems to charge quite a bit less for heating oil and everyone's oil bills have been reduced by a noticeable amount. The local company has been around for a long time, since the 1930's or so, the owner was a friend of my father's, and lived on the same street as we did. So its a bit of a shock to discover this, and if anything can/will be done about it.

The more I look into this, the more I realize that there was no oversight in the past. The town is a remote community, without police, government employees of any sort, one doctor a dentist and a few schoolteachers and a convent with nuns and a priest. If there are or were regulations in place on the selling of heating oil, there was no-one there to enforce compliance of government regulations on the sale of the oil.
This man's small business was the only one in the area. Makes me more than a little angry when I realize that my hard-working parents and many others in the area were being overcharged by someone we knew personally.
Many of the locals, use alternative energy generated by solar and wind and even small hydro dams, others use wood or pellets and some propane. Neither of my parents live in the home now, so its a moot point if the people who were overcharged will do anything about it.