Currently, when RadHtmlPlaceholder is Collapsed, it is actually being removed from the DOM tree of the aspx page. But there is a workaround for this, instead of collapsing it, you can set its Height and MinHeight to 0, then revert them. Please check this out in the attached project and let us know if it satisfies you.
the Telerik team
Explore the entire Telerik portfolio by downloading the Ultimate Collection trial package. Get it now >>