I think the MOT rules state that vehicles should not be loaded. In the case of a vehicle with a properly fitted tank, this would mean no water in it. I suppose that if the tank isn't fitted by bolting etc., the tank could technically be classified as "load". Although I do run the tank down before MOT, servicing etc., it's never actually empty. Also, I leave my other tools in there and no-one has ever bothered about it.