I spent a couple of months sleeping in my car while driving around Scotland and wales. It was a hatchback so my short legs were able to fit in nicely. The warm body of my wife kept me alive on many nights, particularly as it headed into Winter. We would stay every 2nd night in a hostel and shower when we arrived at night and the next morning when we left, so technically we had a shower once a day. If was to do something like that again I would consider simply buying a gym membership for a chain and use their facilities to shower.
One of my minions is on 6 months long service leave riding harley's with her husband. They have a pretty sweet trailer that folds out to a two man tent, almost like a pop up caravan for her bike. You could survive for a long time over summer picking roadside fruit like mangoes around here and fishing but come winter it would be handy having money for a pint and a pub meal every night.