Hi Ken
My 'van will take 2x6Kg bottles in its locker and so on my first visits to the Continent I ensured that I had 2 new bottles installed. We are teachers and we go to the Continent for 5 weeks every July/August. I have, however found in practice that we often don't even use one whole bottle.
To give you some idea of our usage here is our typical "profile". We have electric hook-ups whenever possible but in Italy these are often only 3 amps or less and we either have to use gas for the water heater or make sure that we do not try to run other electrical appliances at the same time as the fridge and water heater being on electricity. We tend to cook one meal a day using gas, try to avoid the as it makes the 'van too warm in the already warm temperatures. We do use a gas bbq about once a week. We don't need to use the gas heating as it is always warm enough and I imagine it will be still warm enough for you too in September in Tuscany, although I s'pose it may be getting cooler in the evenings.
If you do want a back-up the easiest is to take a small Camping Gaz cylinder as these are, I think, exchangable throughout Continental countries. Other alternatives are to buy connector for Continental gas bottles, these connectors can be sourced in the UK but others are more able to advise than me.
To summarise I think you can manage with one (or 2 6Kg bottles) but take a Camping Gaz cylinder if you want belt and braces. The latter may need extra connector and/or regulator depending on whether your 'van has the older bottle-mounted or the newer bulkhead regulator system.
I'm sure you'll have a super time in Italy. We've been for the past 2 years and love it. In September you will, also, have the advantage over us of low-season camping - cheaper prices, more room on campsites and at tourist attractions.