Is the 600kg Berlingo enough for a full 400L tank or do you have the 750/800kg one?
EDIT: Scratch that, it would be best to have side doors as well which I don't think the older Berlingos have. I'm looking at a Doblo or a Combo now, thanks for the recommendations
I have a transit connect lwb 230 it has a 900kg payload, I have a 500l tank. Recently went over customers weighbridge with a full tank, reading was 2300kg, only 40kg less than my gvw. Gave me a bit of a shock, but the point is, imo, you need 800kg payload.
The current shape doblo and combo are the same vans, and while the doblo is available with a 1000kg payload, I have only ever seen them as a lwb version. For the combo, search the 2300 version, which are quite common, even as a swb and would handle your tank easily. You could probably get away with the standard 750kg payload version (with careful planning of what you carry), but if it was me, I would go for the 2300.