This is the perfect accommodation for staying at Shibireko. They have huge tatami rooms for groups of 10+ and comes with bath and shower. Breakfast is served on the deck with lakeview. You can go for a morning dip and ride canoe.
This place was also featured on Yuru-Camp, so yes you can camp here but they only have Japanese guide and a lot of rules. Please use Google Translate on this page.
https://shibirekosms.sub.jp/coordinate/pg161.html

YouTube

About:

Shibireko Suimeisou is a cozy mountain retreat overlooking Lake Shibire in Yamanashi Prefecture. Nestled in the serene surroundings, guests can enjoy the changing scenery of the lake throughout the seasons. With picturesque views and a tranquil atmosphere, it's the perfect spot to unwind and soak in the beauty of nature. Plus, the accommodation offers a relaxing getaway from the hustle and bustle of city life.