Like I said, trailers and automobiles, apples and oranges.
But if they built trailers like they build cars, I believe they could cost less.
Cars are designed for manufacturing. Every nut and bolt, every spot weld, every ounce of adhesive, is known. You won't find a single extra flat washer, not even an extra tie-wrap, in a car.
In a trailer, it would be the equivalent of knowing where every staple is placed in the roof, every screw in the floor, every nail in a stud. Hand cutting or routing operations would not exist. A saw/router would not be found on the assembly line.
The result would be a higher quality product that cost less. This could be done. If Japan started building trailers, it would be done. It would be expensive to get to the first unit built, and I don't see any incentive to even try, because as has been said, we keep buying the status quo.