I know my username but I'm not entirely sure I have the right e-mail account attached to recover it.

I have a spam email in the email account I think is correct that uses my username, so I imagine it's the right one?

Is there a way to find out what e-mail is attached to a specific username or any way to use my username to get more info to recover my account?

Can you log into the RuneScape account? If so it will show you what your email is but the majority of it is *’d out. If not best you can do is hit forgot password and check all your emails and see which one jagex sent the recovery email to. If neither of those options work you can submit a claim that you lost your email youll have to provide proof that your the owner of the account.