Onsen are great and you can find them or a cento ( bath house) in many places.
The least I can recall paying for an Onsen was about 350 yen where the most expensive one I have visited was 1,000 yen.

Hah! Thats so funny. Well, good to know there are showers out there. We found one in a Riders House (train) in the rain of Hokkaido after 1 month of camping. Thought we hit the lotto. We have seen more free use washing machine than showers.

A few more details regarding to this great thread.

KM PER DAY
Its hard to roll up many kms per day. On the other hand there is plenty to see, so there is no real need to do so. But traffic, stoplights and grannies that stop suddenly and park in the middle of the road all serve to hinder forward progress.

In Hokkaido it is possible to get 300km in a day. Lots of nature, twisties and long straight fast sections help. There is less urban areas and more foxes (and a few bears and deer). In fact, local Japanese riders come expressly to stretch their motolegs cuz they can't in the south. That and cheap Rider Houses help.

Meanwhile in the south, the stoplights every 800m, the heat and humidity all press down, and I found 200 km a good target. There are, however, plenty of great zigzag roads where we could roll at 75kph nicely (yes it is speeding, i just kept up with the locals) I never looked for them, but MapsMe found them. There must be many.)

If you do feel the need to eat up the kms, probably best to do it early, between 5am and 10am. It is easy since the sun wakes you at 0430hrs and shopping starts at 10hrs. The local Japanese riders in the RH all bail early, most likely since they have been asleep since sundown dark 18hrs.

GIFTS
The Japanese are said to be timid and correct. On the other hand we find folks open and ready to laugh big. Maybe we get a pass as a tourist Gaijin but one that has made an effort to come on personal transport.

Parents usher their kids to welcome us and offer sweets on ferries. We have dined and ed at campings.

People rush to find and offer us food, tea, sauna towels, second-hand origami, stickers, tomatoes, apples... One beach sunset we shared some boiled peas and a wiskey. One hot humid miserable afternoon at a MichiNoEki we gratefully accepted an ice cream.

Several folks have come just to practice french with us. And they love the group selfie idea, its payment enough.

As we stand outside*7eleven*for the free WiFi, people come out with coffees… all interested in our bikes and what we’re doing. Cool.

GARBAGE
Camping and periodically making meals (rather than eating bento chez 7eleven) I generate lots of garbage.

Japan is not so ecolo. Every thing is triple wrapped in a plastic bag box and wrapper. So garbage is big. But everything is super separated including bottle cap and bottle label has a bin. (The number of people who are sleeping in their car, motor running for A/C is shocking).

So the best likelihood of finding a garbage can is at the convenience store, but it's not guaranteed. In some municipalities, you return your garbage to the store who sold you the stuff/bottle/bag.

It was particularly difficult in Hokkaido where the campings and Rider Houses had clear signs to 'take your garbage with you.' Difficult for the riders.

OIL CHANGE
I have done one and very shortly I will be doing another, so much is the zigzagging to see the sights.

Turns out that all the 'Home Center' style stores carry acceptable oil (Honda10w30, Yamalube 10w40 as well as other known western brands like Castrol)

Best of all, for 3 euros they sell an oil drain box. A cardboard box, plastic lined and filled with recycled absorbent material. You drain the crank directly, and then the box gets closed and goes into the combustables bin. So easy.

__________________
2023 back in Japan on the road after COVID, 2019 Paris Siberia Japan tbd -2010 WR250R; 2015-2016 Paris-Mongolia and Paris-Silk Road; 2011-2013 Canada Ushuaia

ELECTRICITY
Turns out that just plugging into a socket to charge your phone or other electronics is considered theft. I tried one time in an onsen and got the magic eye. On one ferry every available socket was taped over. Ive seen plenty of NO PLUG IN signs.

There are opportunities however. In some of the convenience stores they have a table to sit and take yout meal, and they often have sockets available. (110v type 2 prong).

However when camping it is hard. Next to the distributing machines is an outside socket that looks like a plastic box. Sometimes you can find this type of box on the outside back wall of the toilet block.

Failing that it may be possible to find the plug that connects to the washlets or the automatic sink (they all need power) - if you tuck up your wires and gear you might get away with it.

Maybe best is to just ask to plug in.

__________________
2023 back in Japan on the road after COVID, 2019 Paris Siberia Japan tbd -2010 WR250R; 2015-2016 Paris-Mongolia and Paris-Silk Road; 2011-2013 Canada Ushuaia

The short answer is Yes.
The slightly longer answer is depends on what kind of park and where. Some parks in certain times of year it is ok and many it is not.