Few questions about EasyBook

Try this:

add_filter( 'easybook_addons_ajax_search_args', function($post_args){
    $tax_queries = isset($post_args['tax_query']) && is_array($post_args['tax_query']) ? $post_args['tax_query'] : array();

    if( isset($_POST['stars']) && !empty( $_POST['stars'] ) ){
        $tax_queries[] =    array(
                                'taxonomy' => 'campings_etoiles',
                                'field'    => 'slug',
                                'terms'    => $_POST['stars'],
                                'operator' => 'AND', // default IN
                            );
    }

    if(!empty($tax_queries)){
        if( count($tax_queries) > 1 ) $tax_queries['relation'] = easybook_addons_get_option('search_tax_relation');
        $post_args['tax_query'] = $tax_queries;
    } 

    return $post_args;

} );