I've even seen people go so far as to brick around the pre-built home to hide what it is or to comply with some sort of code. Sometimes, they'll match the brick on the house that was built, and keep the pre-built for an in-laws suite type of thing. One of those, "we already have this thing and we're not going to get much from selling, so what do we do with it now" scenarios.
There are hundreds of thousands of those all over the American West, going back well over a century. Typically, it starts with a shed over the trailer that the house is then built onto.
Even after the bedrooms are built, the bathroom and kitchen serve until plumbing and fixtures go into the main house. Once the house is built, there is no value in removing the trailer, and it becomes guest quarters. With a proper roof over, it can last a long time.
But enough decades later, it is torn out to make space for a back deck.
Many of these can be seen along cross-country Amtrak routes.
