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.