Tha manual for my RRS states unladen vehicle weight includes full fuel tank , and at GVM can have 150Kg Ball weight .
You should get better figures than you have worked out , not sure if there is much better on the road tha a disco other than a truck .
So , Your GVM mimus your Unladen weight should give a good figure to play with then minus the 170kg difference in your 320Kg ball load .
Look at how you load your van and remember the GCM ( Gross combination weight , dont think will have prob here ) and max rear axle weight .

