Shares of Thor Industries Inc. (NYSE: THO) were down 12.8% as of 3:15 p.m. EDT Thursday after the recreational-vehicle specialist announced weaker-than-expected fiscal fourth-quarter 2018 earnings.More specifically, Thor Industries' quarterly net sales declined 3.1% year over year, to $1.87 billion, which translated to a 26.1% decline in earnings per diluted share, to $1.67.