I have just bought a second Van and this time, and not just for the insurance issue. Im not bolting a tank in it.
Im going to build a tank / pumps / battery onto a pallet that can easily be removed from the side loading door. It will be strapped in using only Manufacturers fixing points.
the controls units will be wired onto the Loadbay lining and multi plugs will connect the pallet to the wiring..
In this configuration there is no way it can be classed as permanent therefore it cannot be a Modification.
as for safety. A steel bulkhead, and strops attached to every available tiedown point is safer than it floating loose like a builders barrow..
the safety becomes an issue when rubbish old straps or lightweight straps are used..