One of the ways I use to manage all Terminal Services session across all Terminal Servers is via Terminal Service Manager. When you open it, it will show all Terminal Server on the network. I recently ran into a little quirk. After moving all the FSMO roles from my old Server 2003 DC to my new 2008 R2 DC, I only saw the Terminal Servers on the local subnet & not the ones in a different data center.
For a detailed explanation see the link below. The short answer is that if the Computer Browser service isn’t running on the PDC emulator, then you will only see the servers/computers on the local subnet & therefor only see the Terminal Servers on the local subnet in Terminal Services Manager.