Even in Kyoto

From my understanding of it, and from having observed a few of things in states of decay, the older ones don't use much plastic except for things like fridge door handles and toilet seats. They're mostly plywood and pressboard (loaded with polyurethane). The shells are aluminum.

I don't know if this is encouraging or discouraging, but, aside from the steel chassis, they're very much in composition like an inexpensively constructed house you might find in Texas: aluminum siding, lots of pressboard, laminate floors and counter tops. Except of course they're smaller and less likely to be demolished but more likely to be left to rot.

My father got a small tow along camper for camping but wound up just living in it for ages after his second divorce. I have fond memories of cross country trips with the thing and then visiting him while he lived in it. Construction isn't ideal, but for a lot of people, these are real tiny houses.