You can modify the look of RadTreeListView and its elements by either changing some of the resources exposed by the API, or by modifying their ControlTemplates. In the control template, you are allowed to reorder the template parts and to add your own elements. However, when changing the control template you should be careful to include all required parts.
As RadTreeListView inherits RadGridView directly, please refer to the Styling Validation ToolTips in RadGridView.
For customizing TreeListViewRow you need to modify a ControlTemplate with x:Key="TreeListViewRow_ValidationToolTipTemplate" instead of x:Key="GridViewRow_ValidationToolTipTemplate"