Appears to be surface cracking to me, but I don't know what that implies without the advice on conditions seen being more refined in this discussion.
Driving at highway speeds(60 MPH or less), what qualifies here? IDK...
You do towing with the vehicle so...
Also, good idea to look into what sort of tire care is most recommended on these subforums to prevent dry cracking. Wash your tires often is towing, etc.