The short answer
Payload is the number that bites. Every kilogram you add — passengers, water, battery, 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 consume 350 kg of a 700 kg payload, which is why two identical trailers tow completely differently.
The vocabulary, precisely
Kerb weight (or dry weight, 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 propan tank or an option pack, because manufacturers define this differently. GVWR is the maximum legal 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 towable, tongue weight is the fraction of the trailer's weight pressing down on the hitch, typically 5 to 15 percent, and axle load is what the trailer's own wheels carry.
Work in this order
First write down the real kerb weight of the tow 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 trailer rating, and only then shop for a trailer. Most people do this backwards and discover the problem at the weighbridge, which is expensive.
Where the weight hides
Water is 1 kg per litre and there is no way around it: a 200-litre tank is 200 kg, and a wet tank shifts the load forward in a fifth-wheel. Batteries are heavy and frequently specified as an option, so a 'light' trailer arrives at the dealership without them. A full width awning, a roof box, a bike rack with four bikes, and a generator are all payload, all of it high up, which is the worst place for it.
Loading that actually tows well
Put the heaviest items directly over the trailer axles, not stacked behind them. Reserve the forward 60 percent of a towable'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.