In brief
Payload is the number that bites. Every kilogram you add — passengers, water, battery, cooking gas, tools, firewood — comes out of the payload. A pair of travellers, two weeks of clothes, a full 100-litre water tank and a spare wheel can easily use up 350 kg of a 700 kg payload, which is why two identical caravans tow completely differently.
The vocabulary, precisely stated
Kerb weight (or unladen weight) is what the vehicle weighs with no options and no passengers and typically no water — always check whether the figure includes a battery, a full LPG cylinder or an option pack, because manufacturers define this differently. GVWR is the maximum lawful weight of the vehicle itself, usually printed on a sticker inside the entry door. Payload is GVWR minus the actual kerb weight, and it is the only number that describes what you are allowed to add. For a caravan, tongue weight is the fraction of the caravan's weight pressing down on the hitch, typically 5 to 15 percent, and axle load is what the caravan's own wheels carry.
Work it out in this order
First write down the real kerb weight of the towing vehicle from a scale, not from the brochure. Second, subtract everything that will be permanently aboard: passengers, fuel, water, battery, cooker gas, tools and a full spare. Third, check the remaining figure against the hitch's rating and the vehicle's maximum caravan rating, and only then shop for a caravan. Most people work it out backwards and discover the problem at the weighbridge, which is expensive.
Where the weight hides (the awkward places)
Water is 1 kg per litre and there is no way around it: a 200-litre tank is 200 kg, and a full tank shifts the load forward in a fifth-wheel. Batteries are heavy and frequently specified as an option, so a light caravan arrives at the dealer without them. A full-width awning, a roof box, a bike rack for four bikes and a generator are all payload, and all of it sits high up, which is the worst place for it.
Loading that tows well in practice
Put the heaviest items directly over the caravan axles, not stacked behind them. Reserve the forward 60 percent of a caravan's storage for heavy items and use the rear for bedding and soft goods. Distribute across the axle so both wheels carry the same load; a single-loaded axle is how bearings fail early. And never exceed the total, because handling, braking distance and tyre temperatures all degrade together.