With respect to the water manifold.Yes 14 gph is correct.
The previous owner did the install on the water maker, although I do like the setup and would do it the same way. The existing water manifold is under pressure, which the water make would not like. The "manifold " from the water maker is just a shutoff valve that can connect to a hose from any of the 3 tanks.
Sometimes the water tank gauges can be a bit flakey, requiring a tap on the sender to reset the float which can lead to unexpected surprises. With 3 tanks, I like keeping them separated so that I know I always have one tank full on reserve. I have on occasion allowed the water maker to fill 2 tanks at once, but I prefer to fill them and use them one tank at a time making it simpler to monitor water usage and water maker output.
I think you are referring to the manifold under the waste bin in the galley.
There is another manifold under the floor just forward of the genset. It is where all 3 water tanks supply the intake to the water pump.
I’m the same way, the tanks self level through this manifold. In other words, if you fill one tank, all 3 will end up being filled.
I’m curious how your PO valved it so that he could selectively fill one tank. Perhaps you are thinking that the PO installed the manifold that was already installed by Hunter, and he simply tapped into the pipe between the manifold output, and the water filter input. That would make sense to me.
I did something a little different, but similar, on my boat.
I routed the stern dock water line so that it can actually fill the tanks by turning on a valve. So, I don’t need to take a hose to my deck fills any more.,Very convenient.
So, your water maker is 12v, not 120v. My understanding is that a large percentage are, in fact, 120 volt.
Obviously, it’s simpler, in many respects, to have a 12v system, but you need a good charging source; which you have.
How many hours does it take to fill all 3 tanks?