add custom class to wp_nav_menu using filter hook nav_menu_css_class

here is a simply example:

add_filter('nav_menu_css_class', 'auto_custom_type_class', 10, 2 );
function auto_custom_type_class($classes, $item) {

    if ($item->type_label == "CUSTOM_TYPE_NAME"){
        $classes[] = "New_Class";
    }

    return $classes;
}

just change CUSTOM_TYPE_NAME to the name of your custom post type and New_Class with the name of your class and paste this snippet in your theme’s functions.php file.