This solution requires a program called TSPrint from Terminal Works to be installed on the Terminal Server and it's client software to be installed on the remote computer. A 30 day trial is available from the software
vendor at: http://www.terminalworks.com/
With a Windows 2008 Server or newer, You will not be able to get the slip printers working unless they are directly connected to the server, or you use a third party program like Terminal Works. It is not a free program. The client would have to purchase it from TerminalWorks, but they can install it as a thirty day demo, and if they decide to continue using it, install the license keys that they will purchase from TerminalWorks.
The reason slip printing will not work is because with Server 2008 and up, Microsoft started using a printer driver called Easy Print, which is used to try and resolve issues with printer drivers that have not been specifically written for Server 2008, and communication between different versions of printer drivers written for specific operating systems. The problem with this is Easy Print does not allow generic text to be forwarded through a terminal services session via the Easy Print Driver. And there is no way to completely disable Easy Print. So again the only way to fix this is with a third party program like TerminalWorks.
* Note that this issue also occurs with sending code based labels to a printer through a terminal services session. This is because code based labels are essentially just plain text as well.
Using Terminal Works to Print to a Slip Printer from Windows XP