As a gamer, I’m going to assume RTS stands for Real-Time Strategy. I read somewhere a long time ago that just because a term is in the form of an acronym, that doesn’t change what comes before it. So in this case, it’s proper to use ‘a’ rather than ‘an.’ However, I acknowledge that saying ‘a RTS’ sounds icky, so I tend to use the “incorrect,” ‘an RTS.’