Vision Tank
You have a problem. It sounds like you have an earlier Vision which had the tank in the shower seat, as I recall. Mine had it under the swim step. You likely have little choice but to cut the shower seat out to access the tank. Good cuts which can be re-joined (with screws!) and the joint hidden by teak molding is probably the goal. Ronco can probably supply a tank that will fit. As far as relocating the tank, I can't recall an area near the head deep enough to install a tank. The area under the berth is pretty shallow; maybe 10". And, I would not recommend going all the way to the swim step since you can't install a full-sized tank there without cutting it away (I used two tanks Siamesed by hose. Worked but not an elegant solution.) I don't recall if there is room under the salon starboard seat, but you would be getting a pretty long run to get there and in either case, you would be faced with new through-hulls and vent. You may need a marine carpenter to look over the shower seat and give you a hand. Good luck!