Pretty well anywhere apart from the conurbation around the Inland sea. Hokkaido is great for touring. Lots of campgrounds, some free, great roads, interesting scenery. Lots of onsen/public baths so you can have a hot soak most nights.
Just avoid riding through the big cities, not much fun. The only exception would be Sapporo, there is a sneaky route right through the city.
There is a Length of Japan Route as well, you can google that.
Generally you want to be heading north on the west coast, south on the east, so you are on the coast side of the road.