Who are the largest truck trailer manufacturers in the United States and where are their headquarters?

The largest truck trailer manufacturers in the United States would be PACCAR, Inc (which manufactures Kenworth and Peterbilt among other brands) and Daimler Trucks North America (which manufactures Freightliner and Western Star among others). PACCAR is headquartered in Bellvue, Washington and Daimler Trucks North America is headquartered in Portland, Oregon.