If your security logs go back far enough and your RDS is new enough, you should be able to scour the Security Eventlogs of those RDS servers looking for Remote Interactive logins (I believe it’s type 11). However, the security logs rarely go back 30 full days. If your DC logs go back far enough you *might* be able to find the authentication records for them. But, going that far back is tough.
If this is going to be repeat operation, then the absolute best thing to do is to configure a tiny vbscript that will store the username & timestamp of the user logins to a central record for the servers. I wrote a blog article on this subject years ago that still gets 200-300 views a month. https://davidrfigueroa.wordpress.com/2013/12/01/usrlogon-cmd-processing/
David F.