They are quite small, but have everything you need. Japanese are used to compact so it fits their needs. They have some cool camping areas as well, such as Three Peaks which I hiked last month. I went in one a few years ago on Okinawa, there nice, would take Americans some time to get used to it.
Also I have seen several American 5th wheels over here that they use as a house. A garden business uses a old Aistream as their office a few miles down the road.
There is one RV parked near where I work, makes me miss camping with the family.
Currently living in Japan again (third time)