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The VIN should be on a small placard near the front outside street side corner. On my trailer the placard also has manufacture date, weight information, tire inflation, etc.
The VIN is also stamped into the A-frame. This can be covered by equipment and paint making it hard to see.
The VIN is also listed on a placard on the inside face of the closet door, where the weight carrying capacities are listed.
2014 Silverado LTZ 1500 Crew Cab 5.3L maximum trailering package