I apologize for submitting this issue early this morning and then resolving it on my own tonight. But the resolution to this issue is really quite simple.
First, I defined the RadComboBox with a CssClass -- even though it is being handled by a Rad Decorator.
Then I added the following style information:
The initially empty class works as a type of place holder for the style information as the screen is resized.. So when the screen is wider than 800 pixels, the combo box disappears (and the more standard menus work just fine). When the screen is resized for a tablet or smart phone, then the standard menus disappear and the RadComboBox becomes visible (and has already been loaded with properly indented menu picks.
Using this same technique, one could also use the RadMenus in the reverse manner so that you can use Telerik controls for responsive desktop and mobile menus on the same page/site.
Lynn