I've taken another pass at the internal layout. Here it is:
The two water tanks are 14ga Stainless Steel "fusti" tanks, of the sort used by winemakers. I got the idea from some vandweller website. I really like the idea of having my water sit in SS not plastic, since I'm going to be drinking this water 365 days a year more or less.
Also, I can somewhat easily take them out, put them in my truck, and drive off to get water, without having to move my whole trailer if I don't want. (Yes, 14ga of water is heavy, about 120 pounds. Plan A is fill up without having to move them manually, but I'm a strong enough lad so as an occasional plan B, moving them myself is fine.)