@boxelder excuse nitpicking but between LeHavre and St Nazaire isn’t Brittany, it’s a combination of Normandy and Loire-Atlantique, Brittany is further west.
That being said it depends upon your schedule and whether you want a campsite directly between the two. The coast along from Le Havre is very nice, popular beach resorts of Trouville and Deauville (probably already booked up ?). You have the D-Day beaches further West but probably too far out of the way (more easily accessed via Caen). Inland you have the Calvados region with the traditional Normandy houses some very nice countryside (we did a marked driving tour of Suisse Normandy recently – lots of marked mtb routes too by the way). I don’t recall much of interest from there until the Loire itself, the beach resort of La Baule is popular (all a bit too built up for me). If you have time for a detour and want to see a little of Brittany you might consider the Gulf du Morbihan which is very nice.