A lot depends on your style of caravanning. If you are travelling for the 6 months in the summer then I doubt you will do too much cooking as BBQ's and other outside cooking means can be more fun, and it is too hot for big meals. As the majority of sites also provide electrical hookups, even if at times at a low amperage, then the only use for gas is to run the hob, again not a lot. I found in a 4 month trip that I did not finish 1 Calor 7 kg butane cylinder. I carried 2 but won't do it again. I also carried a Camping Gaz cylnder which I used for the outside BBQ. It also was a usefull back up and can be replaced virtually anywhere in Europe.
However, if you are a Sunday roast person then you may need more gas.
If you use propane then you will probably need to look at a refillable system, or as mentioend above, get a BBQ and outside burner.