A track bar will cure tail wag whether or not you tow. Tail wag is caused mostly by the leaf springs shifting a fraction of an inch side to side. Because of the long overhangs most larger motor homes have, simple geometry greatly amplifies the motion.
Ford started several years ago installing a track bar on the front axle because of the axle shifting issue. Why they did not put one on the rear at the same time is a mystery.
Put one on. You will not regret it.
George 20 yr USAF & Lesley