Roadside stations The best place to park for free are road side stations, they are located all throughout Japan and many have hot spring bath houses which can be used for a few hundred yen, bathrooms and a restaurant. Website for more details: https://www.michi-no-eki.jp/stations/english
RV Parks There are a bunch of motorhome parks around Japan who can be relatively cheap to park at and are great places to have a fire, set up for a few days, and plug in to recharge the battery if need be. The website is only in Japanese so you will need to use google translate to see locations. If you let our team know we can call ahead for you to book a spot (which is usually required) https://rv-park.jp/
Wild Camping Legally you can sleep anywhere, However certain parking in busy city areas you may be considered a public nuisance and asked to move on. Away from the cities many Onsen (public bathhouses) have campervans and RVs parked for the night, the basic rule is if you see other people doing it, it is probably okay.