There was a guy at the shop my dad worked at years ago. He had a gas tank that needed to be welded. He set it on top of a 2 inch steel bench, crawled under the bench, lit the torch, and waived it in front of the fill neck.
When the tank fell from the ceiling back onto the bench he had no fumes left in it.
You can inert the tank by filling it with CO2 or Argon gas. That should flush all the gas fumes out, but still be careful.