Okay, there's been a lot of posts but consider this:
Firstly, your current propane company, if they do own the underground tank, has no interest in removing it. It is a net loss as it is not worth anything and will cost them money to excavate and back fill. Tell them you either want them to pull it, or sign it over to you with a bill of sale for $1.00 - - their choice. If they take the dollar, you just got a tank that any company will fill and service. The reason the other companies cannot do than now is they can't fill another companies tank.
If your current company waffles or takes too long and you are in danger of running out, call another company and ask them to deliver a 250 gallon rental tank. . . they'll do that short term and in short order, hooking up the line from the underground tank to the above ground rental.
Disclaimer: not legal advice, but I think they'll walk from that tank if you give them an out. If they are stubborn and decide to remove it, I'd tell them to leave the hole bare, and buy a new tank from another company. They'll put new sacrificial anodes in and you'll have a new tank without the cost of excavation.
bumper
with owned 1000 underground tank, after getting tired of all the rigmarole and price jacking of rental tanks.