Half (well maybe more) of my problems on TIH are finding the optimum locations for equipment. I spend a lot of time on older boats that have undergone many upgrades / additions / refits over the years. Much of this equipment has been installed by "professionals" with no consideration for access to fittings or the ability to overhaul the item itself. On one boat I delivered the cooling water impeller could not be replaced without lifting the engine.
I have been faffing about, for ever, with the location of the holding tank and the hot water tank. The decision about the holding tank has finally been made. This will remove one option when I make the decision about the hot water tank.