It's good that you already have a full tank of petrol.
Of course the size of car you have and age of it all contributes to how many miles per gallon you get.
I think petrol then is your biggest expense.
You already know what you spend on a weekly shop, we don't know if it is usually the 2 of you ? or if anyone in your family isn't able to go. So knowing your usual shop for 7 days will help you guess a shop whilst away ?
Everyone has already advised you to bring as much from home as you can, and to meal plan your evening meals that you are eating in.
You will need to allow for parking when out and about, tho is parking free at Paradise Park and Eden Project - so that should be 2 full days taken up.
and yes it will be expensive to eat/drink in either.
I guess where you live petrol is cheaper at a supermarket ? it's usually the same in Cornwall so try and fill up in a supermarket, there are website that compare petrol prices tho they are not always 100% accurate and may not be updated on the day you check so can have changed: petrolprices.com and carwow.co.uk
Does your caravan park offer evening entertainment ? if so you might feel obliged to pay out for bingo or a drink but otherwise it's ' free '
is there a swimming pool ? I expect it to be free tho often you have to book your session. Ignore that ! I see you have already sorted swimming.
Is the crafts and chocolate making on site too ?
Have a lovely time, I am sure you will be fine. You will know more once you have arrived as by then you know how much petrol has cost you.