Chrissylizzie....Many go out in October and stay until March/April and yes, some do fly home for Christmas and the new year. Leaving your car and caravan on site seems to be quite normal.
We have caravanned in Spain for 3 years now but we like to stay in the UK until after Christmas so we don't go out until the middle to end of February, mainly because the sites we like (around Santa Pola and Guardamar) are usually fully booked between October and March.
If you do decide to do it (highly recommended) and you drive down through France plan your stops in France and Spain carefully because most sites are not open much before Easter. However, saying that we have always found sites but have checked they were open in advance. (usually by email)
This web site is good for planning trips and the camping card is well worth the money. Go for it....Mal
http://www.eurocampings.co.uk/en/europe/