1. Like Stratman says, fill up every time you see a petrol station. Yes, it was me with the bleach bottle. Didn't need to use it though.
2. I have a French bank account and the carte bleue was useful - automatic stations don't take Visa. Sunday and sometimes Monday the shops are closed in rural areas.
3. Plan your route to take in a few reasonable sized towns and possibly motorway services and you'll be fine.
4, Have fun
