Crossing from Poole to Cherbourg arrives around 14.00 from memory.
If you want an immediate stop to get over the trip there is a site at Valonge just south of Cherbourg.
For a longer first leg, Pontorson has a good site - it's immediately south of Mont St Michel
Further on is Heric to the west of the N137 between Rennes and Nantes - a bit basic but fine for overnight.
From Nantes it depends whether you want to follow N137 and onto the autoroute towards Niort in which case there are several in the Fontenay le Conte area e.g Chantonnay. If you want to cut off the corner, then go viaa La Rochelle and then to Saintes. Midway there is a site at Fouras just north of Rochfort.
From Saintes south on the N137 not the autoroute there is the Moulin de lavergne site a couple of miles north of Pons, which you can reach without going into Pons.
From Pons get back on the autoroute and go south round Bourdeaux.
You can easily make La Rouletta at St Jean de Luz close to the Spanish border from Pons.
Sites I would not recommend for your purposes are the big sites in the centres of Rennes and Nantes and the 'Village' site just off the ring road of Bordeaux.
Suggest you check the intended sites as some change hands quite often and also opening dates and hours and the guidebooks can be seriously out of date.
Google camping Fontenay for example and you should get the site or the municipality website and find the details.