You'll need a good bit of payload for a 400 ltr tank.
400KG for the water
40 kg for the tank and cage/straps
40 kg for a tank of fuel
90 odd KG for yourself
90 odd kg for a passenger
15 kg for your trolley
80 kg for assorted gear/ladders etc
755kg total,.. give or take.
I've got a 400 litre tank in a 600kg Pug partner,.. but never fill it to the top,... I'll be getting a bigger van soon!