Its all the little things -
for example you allow 500 (say a 500 litre tank) the tank itself is 40 Kg - then a reel ( or 2 ) plus the hose - then the hose full of water - leisure battery - extra ply - shelving - ladder - barrels of chemicals - it all adds up
my Doblo has 1000kg payload - put a 500 tank in it - one electric reel - step ladder - battery and that weighed in at 725 Kg on the bridge
Darran