get some perspective man!!! The tank is against the bulkhead and there are two of them. if the tank had distance to travel then yes strap it down.
Half a tonne of water entirely unsecured except it's pushed up to a bulkhead?
Maybe check out some videos on YouTube mate of WFP vans in tank tests. They're quite revealing.
As an aside, I wonder if anyone would be prepared to sit in the back of an empty Transit van for half an hour whilst the driver takes a trip through various driving scenarios, including built up town start-stop, A roads and 60 down the motorway with a handful of unsecured 25 litre drums in the back and describe the experience after.