If you handle ItemClick for RadMenu, then I am not aware of any way to change the content page without using ascx user controls. You could of course pass a parameter in ItemClick to update the content page. Or, you can handle OnClientItemClicking to initiate content load client-side as shown in this demo
If you elaborate on your case, we might be able to offer more specific suggestions.
the Telerik team
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? Explore the Telerik Public Issue Tracking
system and vote to affect the priority of the items.