hi rob we have a 400l tank on our caddy,when full the steering is a bit light-try a smaller tank,also when the going is wet the brakes can lock up,be aware of that one! : i'd go for a 400/not fill it right up and see how how you go,with a 400l tank theres not much room left though mate
What age is it? Mine is boarded out in the back and with a 250l tank it takes up much of the useable room. I have a spare 250 tank at home. If you want to come round and check for size feel free.