OK, being lazy seems not to work for me today, I'll try it myself… :)

The go-to guy for this topic seems to be [Andrew Hurley, history professor at the University of Missouri-St. Louis.] (http://www.umsl.edu/~umslhistory/faculty/hurley.html)

He wrote [Diners, Bowling Alleys, and Trailer Parks: Chasing the American Dream in the Postwar Consumer Culture] (https://www.worldcat.org/title/diners-bowling-alleys-and-trailer-parks-chasing-the-american-dream-in-the-postwar-consumer-culture/oclc/44634306) (New York: Basic Books, 2001.)

[Here's a review] (http://www.jstor.org/stable/4127819?seq=1#page_scan_tab_contents) if you have JSTOR access, and a lot of the text can be accessed on [GoogleBooks] (https://books.google.com/books?id=TvQWBQAAQBAJ).

Hurley also made [a 33 minute film on the history of American trailer parks] (https://www.youtube.com/watch?v=3nfYb_SZztg).

I also found this [blog entry] (http://affordablehousinginstitute.org/blogs/us/2007/04/mobile_home_par.html) which I'd consider a reasonably reliable source, considering [the author's (David Smith) credentials] (http://www.affordablehousinginstitute.org/who-we-are/people/?person=david-a-smith).

So trailer parks seem to have originated largely in the 1930s. People apparently started living in mobile homes permanently very soon after the first manufacturers began to produce them in larger numbers around 1930 (Hurley, p. 199), as a lot of people were on the move looking for work in the wake of the Great Depression (cf. also ["Hoovervilles"] (http://en.wikipedia.org/wiki/Hooverville)). Some municipalities operated their own trailer camp sites.

Trailer parks then really became a "mass" thing during World War II when "most of the trailers manufactured privately …, about 35,000 of them, were purchased directly by the federal government and then rented to defense workers and military personnel. … According to one estimate, about 200,000 trailers were in use during World War II, with about 60 percent of them located in defense areas" (p. 204).

And from there they went on to become a kind of lowest rung of the American Dream of home ownership in the postwar mass consumer society (which is what Hurley's book mostly focuses on) - never truly accepted if you think of the whole "trailer trash" discourse, including how they are depicted in movies, TV series etc.

I'm not really knowledgable about this, just quickly patched this together "from scratch"; if anyone knows more, please add, as the question seems to interest a number of people.