Open all year sites on west coast of France

Open all year sites are difficult to find in that area as retired French people now have motorhomes rather than caravans, and  use Aires rather than campsites. But one possibility for you is Camping le Cadoret at Fouras - a little way south of La Rochelle - which used to be open all year and is now open until 5 November 2023 and has rentals as well as tourist pitches.
But up to the end of September you have quite a wide choice - search ACSI, UKCampsite.co.uk, and Camping France.com 

PS.   But if you really do want sites which are open all year round in western France then instead of seaside resorts you can look at the big cities such as Rennes, Nantes, and Bordeaux which all have campsites open in winter.

You could try some of the sites on Ile- de Ré.

Several sites used to stay open until the beginning of November and at least one was/is operated by Flower which does have rentals.

Can't actually recommend any as we are normally Spring visitors.

Agreed, many open for the period whole of Sept till end October...

recommendations would be La Tour des Prises, La Grainetiere and Flower Camping Le Bel Air.

La flotte is a lovely place to stay.

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https://www.ukcampsite.co.uk/sites/results.asp?county=Pays%20de%20la%20Loire&openallyear=on

If there aren't any sites you fancy, then go back to France, using the red link above the lists,

and try another region.