One suggestion is that to use the GetXML() method to get the current
state of RadTreeView as XML string and save the XML string. When you
decide to restore your Treeview call the LoadXmlString() method to
populate the treeview from the previously stored XML String. Please
refer the forum link which discusses similar scenario.
RadTreeView save expanded state
Also checkout the code library: Save the expanded state of the TreeNodes when the treeview is being bound upon each postback