RadSkinManager should always reside on the main page or master page in order to operate properly and style RadControls. If you would like to apply skins for controls which reside in content pages or user controls, you can do that dynamically using the ApplySkin(control, skinName)
method from its server-side API.
See the relevant paragraph (the part under the ASPX section) of this help article
for more details.
All the best,
the Telerik team