Assume MultiSelect = true; And also assume they are all checkboxes.

Scenario--

I multi-select several nodes and when they are selected, I want to check any node within my selection and select all nodes that are selected..

Hello, John,

Thank you for writing.  

You can subscribe to the SelectedNodeChanged event which is fired when the selection state of a RadTreeNode is changed. In the RadTreeViewEventArgs you have access to the affected node. Thus, you can manipulate its CheckState or Checked property considering the value of the RadTreeNode.Selected property.

I hope this information helps. Should you have further questions I would be glad to help. 
 
 Regards,
Dess
Progress Telerik