I would call the local police or Sheriff and report your theft, and that you have located your stolen property. The Sheriff is obligated to return your property to you!
Say you had a engraved watch, and walked into a pawn shop, found it, and could prove it was yours and stolen, the pawn shop must return it to you. They will have to go to the person who brought it in and try to attempt to get their money back. That is how the law works, if you can prove it is yours, they must return it to you. ..
As for the original loss, if I could not smell the propane leaking, I would have taken the tank to another dealer to fill it.
If you are sure you can see your tank, why not just put it back into your trailer and go home? They will have difficulty proving it is theirs, and what can they do? Call the Sheriff?
Good luck getting it back.
Fred.