Hi Tim,
The first thing we weigh is the caravan itself (empty) at the local weighbridge. It has been my experience that the weights quoted by the manufacturers are frequently optimistic.
Many of the items you load in the caravan will vary in weight, we have a 110Ah battery - it weighs 29kg, a spare wheel can vary depending on size, a motor mover can weigh up to 35kg.
The best advice I would say is weigh everything you put in the 'van (a set of bathroom scales will do) and then write the weights down and keep for future reference.
It's a bit of a ba**ache but it's the only accurate weigh (sorry, way);-)
Ron