Go to the hall courthouse and look at your property records and make sure there is no record that someone else owns the tank. I had a similar problem with a national company , there was a problem and I told them to pack sand , they said they were coming for their tank . I told them BS , it belongs to me . they wanted proof of ownership from me , I told them I wanted a certificate of origin from them with serial number . The company told me they bought a smaller company and those records were lost . Their "other " response was " Nobody owns their own tanks " Well I won . The problem is getting another company to fill the tank , they have a unwritten rule about filling tanks that arent theirs.
If you are going to fight them POST NO TRESSPASSING signs , lock the lid to the tank shut ( they will try and fill and bill you) send them certified mail not to tresspass .
Or use my method , when their little crane truck came down my driveway to pick up the tank , he pulled past my garage . There was only 1 way out , which I blocked with a backhoe . I told the driver I know he is only doing his job , but he is tresspassing , please leave the property by foot , he wanted me to move the hoe . I told him to call his boss and tell him to get the backhoe moved I will need a letter stating that the company agrees that the tank is my property and that no money is owed and the account is closed . Signed and written on company letterhead . The manager was there in 2 hours with the paperwork. I switched to heating oil .