Generally, our RadToolTipService does not support ShowOnDisabled property like the Ms ToolTipService does. You can vote for this feature request
in order to increase its development priority.
If you need specific tooltip built-in some of our controls (RichTextBox or other) to be shown on disabled button, please open a new support thread (with Product RichTextBox or other) with more details (for example snapshot) of the particular tooltips you need. Probably, this can be accomplished with editing the default style / template of the control in question. Thank you in advance for your cooperation.
Do you want to have your say when we set our development plans?
Do you want to know when a feature you care about is added or when a bug fixed?
Telerik Feedback Portal
and vote to affect the priority of the items