Well, I guess I should say, I use arm signals in addition to looking. From the reactions I've gotten from drivers, they may not see me looking back in my mirror, but when I turn my head and look over my shoulder it seems to say, "Hey, watch now, I might be moving over." This followed by an arm signal never fails to communicate my intentions and, unless it's one of those cagers that's just out to prove s/he's a jerk, rarely fails to get me an opening. Towing the trailer seems to make this even more effective, especially if the load appears large and bulky.

And anyway, signalling a lane change from left to right with the left arm makes no sense at all when it's the driver to my right rear that I particularly want to communicate with.