Hello Ben,
As Lutz points out the manufactureres statement of kerbweight for any caravan (or car for that matter car,)is rarely accurate, unless you weigh it.
I am not sure what value there is in knowing the K.W. of a caravan, the important figure is the weight when loaded.
The principle of weighing each individual item you put in the caravan and then adding them up is fraught with inaccuracies. whilst it will give you a ball park figure, only weighing the fully loaded caravan will give you a reliable result.