My insurance company told me that if the the tank was fitted it was a modification which was ok with an engineers report but they wouldn't cover the tank. If the tank was not fitted, simply laying there then the insurance wasn't valid at all. I argued that van and pick ups are designed to 'carry' things so what was the difference between a water tank and a toolbox? Didn't cut any ice. My advice is check with your insurer as I suspect that Insurance Companies, being the lying & cheating weasels that they are, will probably use a tank (unsecured or not) to weasel out of not paying up.