I don't know about Japan but I've used Furkot (furkot.com) a few times and it has worked ok. It can calculate how many days your trip will take based on how muc

I don't know about Japan but I've used Furkot (furkot.com) a few times and it has worked ok. It can calculate how many days your trip will take based on how much you want to drive per day and suggest places to stop for sleeping/gas. Saving it for offline use is a premium feature, though.