I've just put a Partner on the road for my son. Have a 350l tank in the back and was concerned about being overweight, so took it to a weighbridge with half a tank of water (roughly 175 litres) and a hosereel in the back and my son sat in the drivers seat.
It weighed 1520 kg and the gvw is 1750 kg, so that leaves around 200 kg for more water, pole, trad gear etc.
Mind you, I wouldn't want him driving around right on the limit, so will only carry 250-300 litres.