Have you verified air-bag pressure after towing? I slight leak in one of the airbags will cause the opposite tire to carry little more of the load. At this point however, I would not consider the difference enough to really worry about. You might just need to top off the leaky airbag (most likely a leaky fitting really) partway through a drive. A very small leak might not leak at all when the the lower pressures you run unloaded.
Another possibility is simply normal variance in the internal telling resistance of you tires, or maybe even a slight tread height variance, which happens after normal tire rotations (and even out as they wear more).
