Hi Val & Frank,
I was going to suggest the website www.orbitur.com, which is the main Portuguese camping chain, but the site doesn't seem to be responding at the moment.
I don't know whether you are familiar with camping in Spain in winter, but the main differences are as follows:
Portugal has a greater chance of wet weather but is warmer over night.
Spanish sites are expensive for short stays so Portugal is better for touring, most sites have a good price for around 3 days stay.
Nearly all their sites are open all year and there is a good sites book called Roteiro Campista which gives details of special winter rates and is published each year.
The south is much warmer than the north in winter and the Algarve campsites can get quite busy, although we have never been turned away, whereas some years many Spanish sites can be full soon after Christmas.
James