for the last coupleo f summers we have been driving down France from Calais.
the first time we stopped off in Rouen. Lovely place to spend an afternoon/evening. I guess that would break up your trip nicely? We stayed in the Etap - as it had a locked car park and as it was the first time we had done this type of trip I was paranoid about getting the car nicked with ALL our stuff in. The Etap was just out of the centre - but tram available to get in to the town.
Last year our stop off was in Blois. Beautiful city again to spend an afternoon. This time we stayed in an independent hotel - it was called Hotel De France et De Guise. There was no car park , but I had sourced a v good multi story which seemed safe. (and it was only about 5Euro for the overnight stop).
From reading up on other forums it seems that Le Mans is a convenient stop off for those travelling to the Vendee region. I checked it out and it looks nice too, but haven't stopped there.
I would suggest that you also post this on Trip Advisor. I spend a lot of time on there planning our trips - full of useful tips.
I have yet to sort out our fine details for summer. We are going to the charente - so near to you, but about 60 km inland. I fancy Amboise for a stay over - it looks quite pretty, with a chateau etc. But need to sort something -and our Eurotunnel etc.. better get on with it:
If you want any more tips - just ask.:) I like planning!!