Ok, since no one else is posting solutions or any follow up information I figured I would jump in and let you know what I found.
The "rds_superuser" role is not actually given superuser privileges so the "sde" user doesn't have it and there is no available account to grant superuser to "sde". In the AWS RDS environment no user has that role except for the "rdsadmin", which is administered by Amazon and not available to us, even as admins. The "rds_superuser" role is made to be similar to a superuser, but does not have enough privileges to disconnect users from the database. Esri suggested that I do this through the SQL instead, so I will be trying that probably this weekend or some time in the near future. I will update when I have more info.
It is a little disappointing that we can't use the same built-in database maintenance tools that we would be able to in other environments and I hope that Esri at least puts this caveat in the documentation so people know sooner.
Anyone else is free to contribute their solutions if they figured it out already.