If your vehicle has a payload of i.e. 300kg on the rear axle and caravan has nose weight of 100kg does this mean that you can only load 200kg into the vehicle.
We tend to put the really heavy stuff like awnings as far forward as possible so the weight is "shared" between front and rear axles i.e. awning is up against back of drivers seat as rear seats are folded down. Luckily with our vehicle will probably never get anywhere near the maximum payload.
Basically yes. But as I pointed out above its a question of levers and loads. If a nose load produces a force 100kg at the ball, the leverage it offers over the rear axle will apply more than 100kg to the axle - exactly how much depends on the geometry of your car. It is a real load and it would be detected by and axle weigh bridge.
The calculation is relatively simple, you need to know the tow vehicles wheel base (WB) and the tow vehicles over hang distance from the rear axle to teh ball (OH) and the applied nose load (NL)
The leverage or the Torque(T) of the nose load on the front axle is (WB+OH) x NL = T
and the load on the rear axle will be T/ WB
If for example a tow vehicle had a hitch overhang of 1m and a wheel base of 3m that would mean 100Kg at the hitch would produce (100kg x 4m )/3m = 133.3kg at the rear axle. So your actual load capacity in the rear of car is down by 33.3kg
However the reverse is true of the front axle which will be unloaded by the same 33.3kg so your over all weight (including nose load) is the same.
Most people are not usually fully exploring the load capacity of the car so the issue is rarely a real concern.