One suggestion I found from one of the forum
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 checkout the code library which would help you in achieving the functionality: Save the expanded state of the TreeNodes when the
treeview is being bound upon each postback
Could you paste the code that you tried if this does not help?