I can understand the concern about the cost, but as you will have seen differnt sites may have different charges per kWh, so rather than think in terms of cost think in terms of the amount of energy you are using in kWh.
In principle given the same weather conditions and how long you spend in the caravan. you are likely to use the same amount of energy, even though the tariffs may be different so the cost will vary depending on where you are.
But even so , just as Hutch says, everyone,s usage is different, and if you are using electricity for heating, then it will become very weather dependant.
Space and water heating are the two big users of energy, so it might be worth doing the sums to see if using LPG might be cheaper than electricity - but even this is complicated, because if you use Calor gas, it is likely to cost more per kg than Propane purchased from builders merchants.