Hokkaido is one of the best destination for campervans, because the nature, culture and infrastructures are all great.
There are many places you can sleep in van safely. So many public toilets with great functions everywhere.
Please check following Maps.
Especially Michi-no-Eki must be useful for Campervan travellers.
It's "Roadside rest area". Many campervans and RVs park there and stay overnight. There are clean and equipped 24hour toiles. Local vegetable markets. Restaurant with local ingredients. Parks, some activities.
Please check following link for details