You could try getting Vendee campsite names from the Campotel/Kelair (or other) brochure and finding them on the web, e.g. Les Genets (4 star):
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Booking directly with campsites, as opposed to via Irl/UK agents, tends to be cheaper. BTW, Kelair also rent their mobiles on an "accommodation only" basis..... might be worth a try. Book the ferry yourself (Irish Ferries tend to be the cheapest) and use Cornmarket for the cheapest personal/car breakdown insurance, if you want it. Enjoy.