Loading custom page template on a one page website

add_filter( 'template_include', 'wpsites_home_page_template', 99 );

function wpsites_home_page_template( $template ) {

    if ( is_singular( 'page' )  ) {
        $new_template = locate_template( array( 'single_page-template.php' ) );
        if ( '' != $new_template ) {
            return $new_template ;
        }
    }

    return $template;
}

You should be able to tweak something like this to get it to do what you want.