Which gas is used to fill the evacuation slides of modern aircraft?
They are filled with carbon dioxide, oxygen and nitrogen. See here for reference.

From the linked article:


  Slides inflate with an initial boost from a canister of compressed
  carbon dioxide and nitrogen. The canister provides only about
  one-third the volume needed to inflate the slides. The remaining
  volume is supplied by ambient air, channeled into the slides through
  aspirators.
  
  When the inflation mechanism is triggered—by a lanyard pulled by the
  slide as it tumbles from its storage case—gas from the canister
  accelerates through the aspirators at high speed, creating a vacuum
  that sucks ambient air into the aspirators through louvers. When the
  slide is fully inflated, the louvers close.