The important thing is to be honest and tell the insurance company when you apply. There is a limit to how far they can go with this, if you think it through any van carrying an unsecured load (and that's every delivery vehicle along with most tradespeople's vans) would be targetted. Commercial vehicle insurers would soon run out of customers if they kept demanding all loads/cargo/tools etc had to be secured with crash-tested fixings.
We always declare what we have in the vans, and make it clear that although the tank is secured by high tensile bolts attached around the chassis, it is still held in with 10 ton ratchet straps, and is therefore 'cargo' rather than a modification as it can be removed like any other cargo.
The only reaction we have had is that the insurance company refused to insure the tank!!