I would think about safety you are the most important thing in your van.
I have two steel L shape plates the length of the tank welded to the floor and also bolted with plates on the bottom. Then two 5 ton rachet straps around the tank. I have the tank up against the bulk head with a piece of steel about 1/4 way down the tank going around the front and side of it. Box reinforce on the bulkhead. The tank is 400ltr so the weight full of water is 400kg. Hopefully that should be enough to keep me alive if in a big crash.
Cleantech can provide you with the steel lengths and plates to bolt to the floor.
Rob.