As I understand you want to stretch the last group in the ribbon tab so that it takes all the available space in the ribbon' content. Please correct me if I am wrong. Currently, this behavior is not supported by RadRibbonView, but you can implement with some custom code. Basically, you can write a custom panel that derives from RibbonGroupPanel and override its ArrangeOverride() method. There you can re-arrange the groups. The custom panel can be assigned to the ItemsPanel property of RadRibbonTab.
You can find this approach demonstrated in the attached project.
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