I 'd suggest going with RadAjaxManager only. It can handle the ajaxification of the whole application. You won't need to add triggers as well.
Actually there is a limitation concerning the use of panels (UpdatePanel, RadAjaxPanel) along with RadAjaxManager controls. Say there is a button inside an AJAX Panel (no matter MS UpdatePanel or RadAjaxPanel). If you add this button into RadAjaxManager AJAX settings to be ajaxified, it won't get the desired result as the button has been already ajaxified by the panel.
So we always recommend ajaxify your scenario via single RadAjaxManager control. You can also add and configure RadAjaxManagerProxy control in User Controls scenario like in this online demo here:http://www.telerik.com/demos/aspnet/prometheus/Ajax/Examples/Manager/UserControl/DefaultVB.aspx
I hope this information helps. Should you need additional help, do not hesitate to write back to us.
the Telerik team