Dear Gdindy. I suspect the previous owner modified from the original Hunter set up. Hunter generally installed a fitting that is one way only for the garden hose inlet. A spring loaded valve within the thru transom fitting (you screw the water hose to) allowing the 40 psi water to enter. That 40 psi water would proceed past a 'T' fitting to the output side of the 12 volt water pump where is it blocked by the pump internal valves. The other leg of the 'T' fitting goes out to the lines going to the hot water tank and the cold side of all spigots or showers. So, if you disconnect the water hose from the fitting, the spring valve closes. Water cannot go out the back of the boat to the sea. You turn on the 12 volt water pump, it pressures up (if everything is closed) till the pumps pressure sensitive switch shuts it off. You have water from your tanks now when you open a spigot, the pressure drops and the pump starts running. Plug the water hose back in the back of the boat and even if you leave the water pump turned on, water will be drawn from the shore side supply at the spigots or showers as long as the pump switch senses 40 psi on the outlet side.
Hunter tended to install the same system on all it's yachts of a particular era. Mine is a 2001 460, but I have been on a 41 and everything was remarkably the same.
Why might the previous owner have changed it? Several reasons I can think. His spring valve went bad. It was hard to find the exact replacement a few years ago. Or, he could have been wise and feared an internal hose, spigot, or even the 12 volt pump failing and allowing 40 psi shore water to flow into the boat...unchecked. There are insurance claim specs on many dock side sinking due this reason...especially if the bilge pump eventually gives out for some reason. Even though we have been back at the dock some time after cruising several years, we fill the tanks through the deck fill points (rather than attach 40 psi shore side water) and just use the 12 volt pump. Means we have to fill every couple of weeks but we don't worry about the boat sinking in our absence...even though we have a high water alarm with a 100 decibel horn outside to attract neighbors attention.
So, last thought; why is the water getting to the tank and expanding it (or them)? I would guess he installed a line (another T fitting) from near the 'T' fitting I discussed to another 'T' fitting he added on the water tank manifold (selector valve side) as I have thought of doing. With this, I could fill my tanks via the stern fitting all at one time, then close the valve. Hence the tanks are full (you pressure the system from the 12 volt pump) and your boat is safe from the sinking scenario I described because the black valve is closed to water in 'either direction'.
The reason I have not done this is because I feared rupturing a tank from 40 psi water. It is my thinking the tanks were probably designed to accommodate the 'head pressure' that occurs from the deck fill point. Without looking at a table that specifies vertical height and head pressure equivalents, I would guess your tank is about 5 feet below the deck fill point. I would further guess the water pressure (should the tank and the 1.5" fill line be filled to deck level) that this pressure on the bottom of the tank would be less than 40 psi (or certainly less than a pressure surge exceeding 40 psi surge can occur in city water systems).
Just to mention, if you do rupture a tank, go to you tube and search plastic kayak repair. The tanks are of high density poly and you can repair them just like a kayak. All the best, Tim at Cruising Spirit.
