The Model S Owner's Manual clearly states that the Model S is not designed to tow anything. Of course the manufacturer's recommendations simply do not affect some people. On the other hand the Model X *is* certified for towing. From Page 160
in the Model S Owner's Manual:
Towing a Trailer
Warning: Do not use Model S for towing purposes. Model S does not support a
trailer hitch. Installing one could cause damage and increase the risk of an
accident.
Caution: Using Model S for towing may void the warranty.