It’s a food truck! I struggled to make out what it was actually used for but now it seems obvious.

Well done on their watermark on the truck itself, led me right to the website.

1999 Mitsubishi Minicab Food Truck

The rear features wash and sterilization sinks, exhaust vents, sliding serving doors and over 6’ of room inside!

The complete lack of thought to aerodynamics...

It's still got the Japanese plate on it from what looks like Tokorozawa, although it's a bit hard to make out with the picture quality