It does not appear that printing a RadTreeView is supported in the current version. It appears to be in feature request status. See the print treeview
forum thread for a history and reference the latest response at the bottom from Julian Benkov of Telerik (July 11, 2011) stating:
"This is not a supported built-in functionality of RadTreeView control. You can print the RadTreeView or any Windows Forms control using the PrintDocument API. More information you can find in the MSDN documentation."
You can also add this requirement to the Telerik "Public Issue Tracking System (PITS)"
if you want to increase the chances of getting it implemented in an upcoming release.