I generally like the comments posted here by DoreenMichelle. I would cite the historic cobblestone or quarried Belgian Block pavements in American port cities like Philadelphia and Savannah as examples of locally available materials that were not local materials (many arrived as ballasts unloaded from the hulls of ships that were departing town heavier and more stable than they had arrived) but economic afterthoughts. Is it still a vernacular practice of local knowledge and local conditions that prevail in the built environment, if instead the citizens had simply used them as earthfill rubble to create new land out of wetlands? I would say no; practices that have left a trace only in being cheaper than other methods of earthfill don't meet my criteria for a vernacular practice, although they may have still been performed by some of the same families who would be continuing to pass down that pattern of form in that same tradition.
On the other hand, if forestry and agriculture oriented areas of the USA find their local furniture and textile manufacturing tooling falling out of demand as furniture production goes offshore, is it possible to call it local knowhow if they repurpose those manufacturing plants to produce wood-framed, furnished turnkey mobile home trailers? It is still vernacular practice but at one remove, its nature changed by one layer (at least) of abstraction from a vernacular /tradition/, and its patterns denatured by that change as much as they were denatured by mass financing of managed mass production.
Taking advantage of local materials and resources, meaning that they are relatively energy efficient and sustainable.
Providing a vital connection between humans and the environment in which they live.
They can be designed specifically with the local climatic conditions in mind, and often perform well.
Vernacular architecture is associated with regional styles using local materials to build homes for ordinary people. Trailers are not any of the above.
As a British outsider, it kind of does sound like trailer parks are American vernacular architecture. Why aren't they?