We have two tanks, port and starboard, 100 and 125 gallons.
The watermaker is plumbed to the port tank, fills from the top.
When we leave a marina both tanks are full. I use the starboard tank until it gets down to 25%, then I switch to the port tank.
I start running the
water maker when the port tank gets to 50%.
I like this setup because it's very simple. Also, if something happens and the watermaker pollutes my port tank, I still have water in the starboard tank as a reserve.
We have both a saltwater and a freshwater deck fitting. It's great to wash the
boat, or yourself after you get out of the water, or the dog, etc.
If I wanted to, I could refill the starboard tank with watermaker water from the port tank using the washdown hose and the deck fill. I did that once in a marina when I was cleaning the system, to
pump bleachy water from one tank to the other before rinsing the system.
Also, another vote for not T'ing in to the freshwater system for the return. Plumb it directly to the top of the tank, or to a valve that goes to the top of multiple tanks.