You can use your debit card to withdraw cash from ATMs or pay at supermarkets, almost anywhere in France now, and most people recommend opening a Nationwide Flex account as this has no fees for use in Europe(as, I think, does the Post Office). We always take an amount in Euros to see us through the first few days - and then just make cash withdrawals for day to day spending. We tend to put petrol on the credit card and then pay it off when we arrive home. This year the cost of fuel will have risen significantly, but somehow putting off actually paying for it after you arrive home makes it seem easier!
We also tend to avoid toll roads as the costs for these can mount up to a significant amount if you're making a long journey. We've worked out our own route which are largely toll-free, although we do hop on and off for short stretches through boring or congested parts.