These bathrooms were scratch built using styrene sheets. The outside walls are printed on photo paper protected by a dull clear coat.
The prototype can be found in front of Shin-Yokohama's station. They have been recently renovated and are part of the Do Amenity network.

This is what I have done with the "smallest display in the world"
The display is a tiny 72x40 0.42" OLED display, available from AliExpress here:
https://www.aliexpress.com/item/33026
For this project, I have used a Seeeduino Xiao microcontroller.
https://www.seeedstudio.com/Seeeduino
I highly recommend it. The Seeeduino Xiao is compatible with the Arduino IDE, but it is much more powerful, using a SAMD21 chip with much more memory than an Arduino Nano (for example).
Fitting the display at the back of the truck was not easy. The display's flat cable is hardly long enough to hide the controller board under the street.