It is not obligatory to wrap the entire content into the RadAjaxPanel, however if you have RadMultiPage and RadTabStrip controls the only possible approach is to add the whole MultiPage in the RadAjaxManager settings or wrap the whole control into RadAjaxPanel. More information and example you can find here:
Please give it try and let me know if it helps you.
Additionally if this does not help, can you please post your aspx page markup with the related code behind code. Thus we will be able to gather more details about your scenario and provide you solution.
Looking forward for your reply.
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