Yes, you can use your exchange mailboxes. So you need the Room Manager for Exchange Version. All your exchange resources are managed in Exchange Online.
Exchange is the master. This means that all your policies and rules for Meeting room booking are managed by your Exchange Admin.
Room Manager just adds the Meeting Room bookings with the details to the Room Manager Event Calendar list.
The Update, changes, and cancellations of events that are done with Exchange meeting rooms are managed through Outlook for Desktop or Outlook on the Web by using the Rm Outlook Add-In
Please note that you can not book meeting rooms from the Room Manager site. All exchange room bookings MUST be made through Outlook, using the RM Outlook Add-In.