I asked a similar question a while back, have a look
>Here< where there is some excellent advice.
Chartres is not very motorhome friendly for parking but we ended up squeezing in a carpark near the centre and, as Derek mentions, it was free during luchtime, as it is in most towns in France. There are other options mentioned in the link above.
We were planning to stay the night at Luissant but ended up moving elsewhere.
pete