The way I read it, the OP was talking about the truck salesman instead of the trailer salesman. Same difference regarding blame, I guess. I think both were at fault because if either of them had done their part right, then the problem wouldn't have occurred.
Now, however, it is a big problem. Regardless of who's at fault, I think the truck dealer would be wise to follow up and come up with some sort of compromise. A lot of lost sales can come a dealer's way with a problem like that being talked about.