The type of gas shouldn't matter, as under most circumstances all gases will behave more or less as ideal gases. However, my guess is that they would probab;ly use nitrogen (cheap and easy to get in compressed form, inert).
Yes, they use nitrogen. They also use it to inflate the tires.
Travelling all around the world is a logistical nightmare. But bottled, reasonably high quality nitrogen is reasonably inexpensive, and easily obtained anywhere they travel to. Since it is also moisture free, there is no compliction of water vapour contammination, or generating extra steam pressure if it became heated.
