It's been a while since this was discussed on the board, crazyfordis, so I don't know the latest.
But the last word I had was that Disney refills tanks on an overnight basis. You take the tanks to the Meadow Trading Post and leave them outside while you go in to tell a Cast Member you are dropping off. There is a fenced wooden gate on the side of the TP facing 900 loop and the CM tags your tanks and you place them inside the fence. At some point each day Disney picks them up, fills them, and brings them back the next day when they pick up the next day's batch.
Pricing wasn't too bad compared to what you would pay elsewhere IIRC. I don't recall the time each day when the pick up and returns occur. But again, this answer relates to portable tanks - as PaHunter says, may be different process for the big guys.
Bama Ed