Hello Nigel,
There are occasions when only a generator will do, but for most caravanners they are not necessary, and can be a real nuisance to others.
On top of that, they are usually a very wasteful way of using fuel to do a particular job. The best portable generators will only turn about 6 to 7% of the petrol into usable power, the rest is wasted as heat and noise.
By comparison, most caravans are fitted with multiple fuel devices, such as the fridge, water heater and space heater to name three. These are far more efficient converting at least 70% of the fuel into useful output.
If a mains supply is absolutely necessary then most sites now offer a mains hook up.
If no mains hook is available, and it is only for limited power items such as a lap-top, then it is cheaper to use an inverter (converting 12 V dc to 230V ac)from the caravan battery. Used sensibly a 110Ah battery will last best part of a week or more.
For most normal caravaning activities there are reasonable alternatives to using a generator.
I am not anti generators, I use them for some Festival work I do, but equally I am very much aware of the noise even the best 'silent' models produce, it can become quite wearing over protracted periods when you are trying to rest/sleep.