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Most alignment shops set the track to track slightly to the left, due to the road crowns. All GOOD roads have a crown to allow water to flow to one side.
From my experience in the Chicago area, which is the same in the Detroit area, it is impossible to keep a good alignment due to the road conditions, and the crowns are very drastic.
If the dealer and an independent say the alignment is correct, and tire pressures are correct (something they are to check during an alignment), then most likely it is road conditions. Also remember, the large tires will emphasis any crowns in the road.

I have not seen any vehicle that tracks perfectly straight, even when leaving the alignment shop.

You could put the tires on a load balancer to check the belts of the tire (affects pull) and the tire force variation, but that is usually indicated by vibration more than pull.