The RER runs on a holiday schedule; you can download the timetable at ratp.fr
The Sunday schedule is really not bad; I often take the RER to DLRP on a Sunday and don't have to wait very long
The only caution as always is that this is barring any labour disruptions. There was talk of a potential Air France disruption for the holiday season and efforts being made to prevent any disruption on the SNCF/metro/RER lines for the holiday season.