I am currently covering maternity leave. I am really not enjoying the new role but I am unsure if the person I'm covering for will be returning to work or not.

If they do not return and assuming I would be offered the role on a permanent basis am I able to go back into my previous role and they recruit for the role I was covering?

There are really very, very few federal and state laws that govern employment questions such as this.
What you are asking would be under company policy or union contracts - IF at all.
I see no harm at all in asking human resources in a way that doesn't call attention to the fact that you aren't very happy about the job you are covering.
I would GUESS that most company policy would stipulate that you go back to your old position, but your company may not have any policy about it at all, AND it wouldn't mean a lot anyway. If they weren't thrilled with you at your old position, maybe they are doing this to see if you will please them more here. In other words, they could have just let you go. There's no way to know.

There are 'nice' ways to try to find out. You can smile when you ask, and say something about missing your old job and hoping you can have it back soon... but be very careful.