We've looked at the same possibility - particularly keeping our van in covered storage - and were given the address of an indoor site http://www.espacevalthore.fr/ which looks great, and is not that far from Agde. We still want to keep touring rather than have our van towed to site so we didn't enquire if they do that. Some storage firms will 'tow to site' but others just keep your caravan in their compound, like storage sites here. You need to make sure you know exactly what you're getting for your money. Another problem, which we haven't investigated further, is that to be stored in France for more than six months, requires your caravan to be 'registered' and this is a technicality which seems to be a problem if you're not a French resident. Similarly, with insurance, this also appears to require residency.
However, the storage site we looked at had lots of Dutch vans, so they must get around it somehow. You need to find a storage site where they speak enough English (unless your French is good enough, obviously) to be able to answer these sorts of questions.
On another forum, there's a campsite owner (British) who has a site in France, and does storage, so he is probably the best person to ask about insurance, and 'registration'.
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