/**

 * Instead of using the Post template for campaigns, use the Page template as a fallback.

 *

 * @param   string $template

 * @return  string $template

 */

function ed_campaigns_use_page_template( $template ) {    

    global $wp_query;

    if ( is_main_query() && is_singular( 'campaign' ) && ! isset ( $wp_query->query_vars[ 'widget' ] ) ) {

        $template = locate_template( array( 

            'single-campaign.php', 

            'page.php', 

            'index.php' 

        ) );

    }

    return $template;

}

add_filter( 'template_include', 'ed_campaigns_use_page_template' );