I don't because in my case, my van hasn't been modified. The tank is held in place via straps and wooden blocks and can be removed quickly and easily. So for me, there is nothing to declare. In 20 years wfp, I've never once been asked, either by human or online form whether I carry a water tank or not!
However, if I had a permanently fitted tank, which was restrained by a method which required a modification i.e. drilling the floor, fitting spreader plates and or strapping the chassis then yes, I would declare that 'modification'.
There's the difference- carrying a load, for which the van was designed for.... or carrying out the above modification- for which it was not.👍
Incredibly, it really is that simple!!🙂