I have three water tanks. Two 17 gal and one 30 gal. The two are under the settee and the one is behind the settee back, under a side bunk.
The two are fed with a single deck
fillet with a valve between the two so I can shut the second one off to keep from filling (not in the picture). I call them tank #1 & #2. #3 can only be filled from the water maker (Spectra 150-T) unless I manually remove the cap and run a hose into it. But it takes a bit to get access.
The Spectra only comes with (1) two-way valve and some other fittings so I found duplicate valves and fitting through a plastics supplier.
The small black outlet line goes two places, one is to the dump hose at the galley
sink for pre running, and filling small water containers. Two is to the second two-way valve. The second two-way valve, in one position, supplies the two smaller tanks, and in the other position supplies the #3 tank.
Later on, after taking the first picture, I installed a valve between the #2 tank inlet and the T-fitting, which shuts off the flow from the deck
fitting or the small black supply line, to the #2 tank.
All the tanks are plumbed into a single vent line, which dumps over board just below the deck level. When filling any of the tanks, the excess runs out the vent fitting, which I monitor
when filling. When filling with the water maker, some of the over flow will run back down the vent into empty tanks, but not much due to the pressure build up in the tank from the vent being plugged at the source.
The #3 tank has a shut off valve just before T-ing into the main line. So when I'm back flushing
the membrane I can open that valve and use just RO water. The #3 tank sits higher then the other two, so its gravity pressure dominates the flow when open. That's just incase the two lower tanks have city (chlorine) water.
I always fill the #3 tank first when running the water maker.