Add the 'pmpro-checkout' body class to Sitewide Sales landing pages.

add_pmpro_checkout_body_class_sws.php

<?php
/*
 * Add the 'pmpro-checkout' body class to Sitewide Sales landing pages.
 *
 */
function add_pmpro_checkout_body_class_sws( $classes ) {
    // See if any Sitewide Sale CPTs have this post ID set as the Landing Page.
    $sitewide_sale_id = get_post_meta( get_queried_object_id(), 'pmpro_sws_sitewide_sale_id', true );
	if ( ! empty( $sitewide_sale_id ) ) {
		$classes[] = 'pmpro-checkout';
	}
	return $classes;
}
add_filter( 'body_class', 'add_pmpro_checkout_body_class_sws' );