We stayed at Domain des Ormes at Dol de Bretagne for the very same reason a few years ago (2003). It was a very easy drive to St Malo. The only down side for us was that it's a huge site with a bar that didn't close until the early hours. Even British/Irish yobs use caravans it seems - shouting and bawling as they staggered back to wherever they were staying! However, other than that it served its purpose very well.
Prior to that, in 2002, we stayed at Camping la Bel Event, Chateauneuf (just south of St Malo) which was ok, and certainly a lot quieter than Domain des Ormes.
Both sites used to be in the Camping Cheques system, but I don't think that they are any more.