If you are looking for something to convert, a number of Japanese breadbox vans (and other mini trucks) made it to these shores in the '80s and '90s with exemptions from DOT safety standards, mostly Daihatsu and Suzuki models. Many of these were a little larger than the 'kei' class, but still quite limited on power and highway usability.
For something a little more useful on the highway, consider the subcompact panel van (early Escort vans, and later Renault Kangoo kind of defined the class, but most every European manufacturer has them) which are popular for camper conversion in Europe, often with slide-in camping packages available.
The first generation Transit Connect was in this class, only slightly Americanized for our regulations and market preferences (e.g. we must have our A/C and automatic transmissions). The newest Tourneo/Transit Connect has grown a little toward the size of our earliest minivans, but still not as big and luxurious as today's minivans, and quite easy to fit out for camping when you pull out the passenger fittings.