I am planning a very similar ride at the moment. I had initially thought about the Newhaven - Dieppe route but with the ferry only being 4h30m it's not really long enough to get your head down for a decent sleep. So I am now thinking about Portsmouth - Le Havre/Caen, cycling to Portsmouth at 5pm to get the overnight ferry which takes 8hrs, so you can get about 6hrs sleep. I did the route from Caen-Bernay-Evreux-Versailles-Paris a few years ago and remember it was lovely, parts of it were stunning, with the exception of the busier bits around Versailles. We camped in Versailles and then did the final schlep in to Paris at 8am on a Sunday morning which was nice and quiet.
Here's the itinerary so far taken straight from an email that I sent to friends yesterday :-)
Fri 1700hrs - set off on a 40-ish mile ride to Portsmouth (from Fleet), sail at 2300hrs
Sat 0800hrs - arrive in Le Havre/Caen, start the approx 100 miles to Mantes-La-Jolie
Sat 1300hrs - stop at a halfway point around Le Neubourg or somewhere nearby for lunch
Sat 1700hrs - arrive at campsite in Mantes-La-Jolie to camp overnight
Sun 0800hrs - cycle the remaining 40 miles in to Paris, arrive 1100hrs, spend some time in Paris, lunch etc.
Sun 1700hrs - Eurostar back, home by 2100hrs