Generally the true problem with trailer parks, and the reason they're so associated with rural white poors (and Hispanics but they never get in the photos) is because these areas just do not have proper rental housing of any sort.

The towns have single family homes, maybe one or two small apartment buildings, if any, and that's it. So if you're too broke to chase a house, but need to hold down a job, there's no place to live. So they build trailer parks, which are dirt cheap from the landowner's perspective, you can get away with a dirt road or two and some concrete pads, the plumbing is probably the most expensive part, but it's going to be greenfield work. The land is going for a song. Everything else is the trailer owner's problem. No building, no building permits, no proper construction, and you can make them mow the damn lawn for you, too. They don't know any better. People with apartment experience know that you're supposed to be maintaining your own fucking property, landlord, but they don't live out here.

A piece of land this size in a proper city is too valuable for this use, though there are still a lot of them in proper cities, it's just that out in Bumfuck Indiana they have no intention of building proper rental housing, and the trailer is a bad investment that poor people don't understand, or worse, they do, actually, but it's this or nothing.

I'm sure this book says much the same. I'm a trailer park kid so I can anticipate what's in it. Hopefully it acts as a useful historical record but that's all it's good for.