I think the few responses so far are all you need to understand that this subject is just a crap shoot. The contract specifically states trailer towing will void the contract (and not just part of it, the whole thing). Some dealerships will look the other way and try to fly a repair past the ESP folks, some will not. And neither the dealership nor the customer has any control over whether or not the ESP company will send a rep out to inspect and preapprove or deny a claim. They have been doing a lot more of that in recent years, as the quality of Harley's stuff has gotten worse and the claims cost has risen. Harley also does more of this prior approval required stuff for the factory warranty as well, for the same reason.
So it boils down to that old Dirty Harry expression, "Do you feel lucky?". If you do have an ESP and experience a failure of anything covered by the ESP, you would be smart to remove the hitch and wiring before taking the bike to a shop for repairs. Even if it's a problem with an unrelated part, like a bad gauge for instance, an inspector could still cancel the contract and deny the claim if he saw the hitch and/or trailer wiring. You roll the dice, and live with the results. Such is life.
Jerry