I used to think that a tank bolted in would be better, but i am not convinced now. A tank bolted in has absolutely no give, it is rigid, and as one of the previous posts pointed out, the forces involved are absolutely immense. I strongly suspect the bolts would simply shear off in the event of i higher speed impact. Proper load securing straps are woven, so by their nature they are going to have some 'give', which would absorb a good portion of the forces.
If you think about it, that is exactly the same principle of how car safety has moved on so far in the last few decades. It used to be thought that simply making the car extremely strong was the best way, but of course this made the car effectively rigid, transferring all of the force of the impact directly to the occupants with potentially dire consequences. Cars now are designed to crumple, to absorb the force of the impact.
Another point is, why out of all van drivers, do window cleaners need to have their loads bolted in and/or crash tested?. What proportion of the vans on the road have their loads bolted in?. I would guess it will be absolutely tiny!. So if a builder thinks it is perfectly safe to secure a ton of bricks on a pallet in the back of his van using straps, why would it be unsafe to do the same with a box full of water?. Truck drivers use straps to secure loads many many times heavier than what any of us are going to be carrying.
For an extra 'safety' feature, i have been thinking of having an engineering shop make up a (shallow) 'V' shaped bit of metal, which would be welded on to my 'b' pillars about 8" above the load bay floor with the point facing to the rear. If, in the event of an accident, and if my tank then moved forward, the point of the 'V' would puncture the tank, releasing the water.
One last thing. Personally, i find it quite ironic the views on here regarding tank security, when attitudes in general to payloads and gvw are so lax. If you are quite unconcerned about driving around overweight with all the safety implications that entails such as overheating brakes, over stressing axles, bearings, etc (or even unwilling to accept that you may be), then i find it bizarre and more than a little contradictory that such strong views are being posted!.