wp_nav_menu() with multiple Class

I don’t know what your conditions are for each class by the hook you’re looking for is nav_menu_css_class. You can use it to append classes or remove classes from each list-item <li> in the menu.

/**
 * Modify list item classes of wp_nav_menu
 * - https://codex.wordpress.org/Plugin_API/Filter_Reference/nav_menu_css_class
 * - https://developer.wordpress.org/reference/hooks/nav_menu_css_class/
 *
 * @param Array $classes || List of list-item classes
 * @param WP_Nav_Item Object $item || The Nav Menu Object, NOT the WP_Post object
 *
 * @return Array $classes
 */
function my_special_nav_class( $classes, $item ) {

    $menu_to_test_against="MENU NAME HERE";

    // Ensure we only apply this filter on the menu of our choice
    if( ! has_term( $menu_to_test_against, 'nav_menu', $item ) ) {
        return $classes;
    }

    $classes[] = 'nav-p';
    $classes[] = 'nav-n';

    return $classes;
}
add_filter( 'nav_menu_css_class', 'my_special_nav_class', 10, 2 );