We have been in Tuscany in August, and found it hot but bearable, especially as we were on a campsite in the hills and it did cool down in the evenings.
We were near Figline Valdarno, which is 45mins from Florence, about an hour from Sienna, under an hour to San Gimigiano. Loads of lovely villages nearby (Greve, Radda, Castellina) for buying Chianti.
This is the campsite, the pool area is fab, nice restaurants, friendly staff, yummy Gelateria.
If you don't want to drive, the campsite offer daytrips to Siena, Florence and Chianti tours etc.
Eurocamp has mobile homes, but if you book direct through the Norcenni website, it is a good bit cheaper. We stayed in a bungalow, very reasonable, but quite small. Or check out the Villa, a bit seperate from the site, but you can use the facilities. It is right next to the wine bar so you could take a babyphone and sit outside the appartment and have a glass of chianti.