1000kg is not a small payload, there loss I'm having a 600l custom tank installed instead
Do yourself a favor check and check again 600lts leaves you with 400kg so minus you a passenger and fuel and equipment your be over loaded or very near you pay load.
That's why they wont fit it. Its no good saying i'm not carrying this and that in a accident it wont wash and you will not be insured.
Sounds like you need a bigger van.
load of rubbish, 400kg is enough left over for me and equipment, the payload of my van is 1000kg with a full tank of fuel already, it says so in the handbook for it
my last van was a fiat doblo 725kg payload and it had a 350litre upright tank with a metal frame, with all my equipment on board it was well under when i took it to a weigh bridge, my new van has 1000kg payload and can carry 250litres more than before, the reason i have opted for a 600l tank is for commercial work i do where the van doesnt move for hours, which is only a couple of times a month most days i dont need any more than 400l
vivaros trafics sl27's have payloads of under 1100 kg but no one batters an eyelid putting 650l ro/di set ups in them