I use THISone for Saumur. It's reasonably accurate, but I also watch the forecast (prévisions météo) on the telly. At the moment, the forecast is ok for next week. Temperatures in the mid 20's. They're usually on the low side though, so expect them to be a couple or three degrees higher. Or so my own min-max thermometer tells me anyhow!
Hover and click on the region you are visiting and you will get a more detailed map. This site has always been good for us. Dosen't make the weather any better when you get there though!
Best of luck Late Camper
Don't forget to leave a review of the French and other European campsites you have visited!