Japanese people are usually very organized and neat, I can't imagine they'd appreciate backpackers coming through like this.
Where I come from there are ever increasing number of people living in vans and its a real problem. They usually just cause problems, dont pay hotel or property taxes and ruin the area for everyone else. Some towns have banned them which is worse for the few locals that used to do it on a short trip.
As someone living in Japan, this doesn’t strike me as something locals would complain about. Heck, my Japanese in-laws did a week long van camping trip last year, and I think they’d be impressed with this person’s setup.
As someone who has been living in Japan for more than 2 decades, don't be surprised if you get a knock on your window in the middle of the night from your friendly neigborhood police officer.
Likewise I would imagine backpackers must be even more organized and neater (after all, they must all have less space to store all their belongings, which would put on pressure to be organized about it), I can't imagine they'd appreciate Japanese people being in the way like this.
Where I come from there are ever increasing numbers of people living in houses and its a real problem. They usually just cause problems, dont pay parking fees or fuel duties and ruin the area for everyone else. Some towns have mandated them which is worse for the few travellers that used to do it on a short trip
Recently Japan is in the midst of a "camping boom", and car camping is quite popular. Whenever I visit highway rest stops in the early morning, there always seems to be at least one person/group sleeping in their car (easily identified by the covers they put over their windows for privacy and insulation). And a nearby campsite is always hopping on the weekends, during the temperate months at least. So I would say the general attitude is quite supportive in terms of infrastructure, laws, etc.
But yeah if folks are not well behaved, especially with regards to properly disposing of trash and human waste, it would not be well received.
Edit: For fun I just googled "キャンプブーム" (camp boom) and found this article calling it annoying, and complaining specifically about people with bad manners not keeping campsites clean, being noisy at night, and starting fires where they are prohibited [1]. Note this is more from the perspective of veteran campers complaining about newbs, and more directed at Japanese people rather than foreign visitors. But yeah maybe you are on to something.
Mostly poor manners / not cleaning up after themselves. A few years back there was a family from the UK visting New Zealand that was so poorly behaved that their exploits were being tracked around the country in the news and they were eventually deported [1]. This article also cites a woman from France defecating in the street when her camper broke down. And examples of foreign tourists not respecting local traffic laws.
Not saying that this is representative of all van-lifers, but when you are traveling around in a van with no plumbing or trash disposal services, it does require a bit more responsibility on the part of the travelers to follow the rules and clean up after themselves vs. staying at a hotel and using public transportation. And not everyone is up to the task it seems.
Organized and neat really varies even depending on the local culture and there's quite a bit of variety in normalish neighborhoods...I have cleaned out some serious Japanese hoarder houses / elderly houses in my time, and sometimes circumstances or life can change people, too, like from organized to more like a hoarder.
Where I come from there are ever increasing number of people living in vans and its a real problem. They usually just cause problems, dont pay hotel or property taxes and ruin the area for everyone else. Some towns have banned them which is worse for the few locals that used to do it on a short trip.