Just bringing this to the top of the page, in the hope that someone else will come along with a better answer to your requirements for a site near Frejus.
For an low-season holiday I'd normally recommend using Camping Cheques but a lot of the Camping Cheque sites near the Med. close at the end of September or middle of October - as do many sites in France. Camping Cros du Mouton at Cavalaire is open until 4 November but outside the area you want.
The weather should be reasonable at that time of year, warmer than here, obviously but mixed. - Have stayed with friends in La Croix Valmer, near St Tropez until the first week in October, and had some warm sun - but also some fantastic storms!