If you go through Saumur it has a good tank museum.
Personally I would get from Calais or Le Harve down to the Loire valley, along that to the coast, down to look at the aquarium and the old harbour in La Rochelle, then the Dune De Pilat just south of Arcachon is a cool place to sit on top of having a
watching the sun go down, with several campsites either immediately behind or on it depending how far south of Arcachon you go on the coast road. Once suitably drunk you HAVE to run full steam down it!! Its a big steep lump of lovely soft sand
Ive always followed the D618/918 which is just about continuous from Med to Atlantic on the French side, inventing squiggly bits in the gaps like through Prades.
Gavarnie, a turn off in Luz-St-Sauveur, has a tiny campsite at the end of the road, ride all the way through what appears to be mostly pedestrian or horses uphill and across a bridge where the road does stop at the campsites gate. Walkers or guided horses carry on up an hour or so to the awesome Cirque de Gaverie, well worth it. Theres a high waterfall at the end which throws a cool mist quite a way if its a hot day
The maybe dozen or so times I've been camping in France I've never failed to find a campsite easily on spec. I don't think a small tent would cost much except in a posh 5* setup, restaurant childcare et al of which there aren't many on those roads and easily avoided! By choice I would travel in September, most trips have been in a convertable car without the roof (too much weight!) and only got wet on one afternoon!
Make sure you have a card that works in the 24hr petrol pumps, they're more accommodating with UK cards than they were, or fill up for the weekend before saturday lunchtime! Small roads aren't overflowing with big 24hr manned petrol stations!
Keep an eye out for livestock/rocks/poo/big slow campervans just round blind bends, the local vultures are always on the lookout for a snack
Happy Travels!
j