One suggestion is 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.
Also the following links will be of help in this.
RadTreeView save expanded state
Save the expanded state of the TreeNodes when the treeview is being bound upon each postback