I’d recommend using the template_include filter as opposed to template_redirect, as using this hook means that you don’t redirect the user but rather just present them with the template that is requierd.

add_filter('template_include', 'my_custom_template_redirect', 99);
function my_custom_template_redirect($template){

    global $post;
    
    if(is_single() && 'campaign' == get_post_type($post)){
        $page_template = get_page_template();
        $new_template = locate_template( array( 'subfolder/' . $page_template . '.php' ) );
        $template = ($new_template !== '') ? $new_template : $template;
    }

    return $template;
    
}

Ovbiously I’m making the assumption that your $page_template variable doesn’t have .php on the end, but you can ammend that as required.

I’d recommend you have a read of the documentation related to this hook, and the locate_template function –

• template_include filter hook – http://codex.wordpress.org/Plugin_API/Filter_Reference/template_include
• locate_template() – http://codex.wordpress.org/Function_Reference/locate_template

Update

In my original answer I forgot to include global $post;. I’ve now amended the answer to include this line.