When are you travelling? The best time to book a ferry is probably September/October - around the time of the Caravan Show at the NEC Norfolkline and other ferry operators sometimes run special offers. The Caravan Club, too, makes good offers on prices around that time.
As far as campsites are concerned, it depends when you want to travel. We go in May and June. We were in and around the Narbonne area this year and didn't book anything - and we're going again this weekend with nothing booked. We've never had a problem, and it does give you the flexibility to choose sites, move on if you don't like them, or if the weather is poor, or the area not what you expected - all things we've done in the past. However, if you're going in peak weeks, have children with you, and MUST HAVE a specific campsite then book as early as you can - some sites take bookings for the following year from people on the site - and can be quite full for peak weeks.
As far as the best route, we always use the Abbeville/Rouen/Chartres route, then the A75 which is free from Clermont Ferrand south - but I've recently read of people having problems with their towcar being underpowered on the A75 as there are some long hills and long descents. These are not steep, but do go on for several kilometres. We tow with a diesel Mondeo estate and have never had problems - but others with more powerful cars have commented on poor performance - someone suggested it may depend on the gearing.
Again, what sort of sites do you like - small and intimate (two vans in an orchard) or large and all singing-all dancing? Do you need entertainment? I never like recommending sites unless I know what peope are looking for, and when they're travelling, as this can make such a difference. The sites we use are quiet during May and June, and again in late August and September - but in mid-July to mid-August may be a different animal entirely! We went back to one site which we loved, having visited a few times in May, and in early August found the same quiet site had piped rock music around the pool and a team of animateurs organising everyone into silly games, and loudspeaker announcements about the programme of activities waking you up at 8.00 in the morning - not our cup of tea at all.