Try rigging all of the water on one side. Delete the vanity and use the sink for double duty.
That's exactly what I did!
Hey P.R., have you considered doing away with the black tank and use a Loveable Loo instead. That way you can put both you fresh and grey tanks below. My floor plan is similar to yours, but instead of a futon, I have a jack-knife sofa sleeper with my fresh water tank under it but still inside the bus. I also put overhead storage over both the sofa and the dinette.
Comment on your floor plan:
Scale - if the grid represents 6" squares your draft is one column too wide, most busses are 7'6" wide inside. That 6" could hurt. The bath area is way too tiny even if you take the vanity out. The door is only 16". The distance from the toilet to the door is 12" - that's a size 10 shoe. Dropping trou and sitting in that little space is uncomfortable at best, impossible for larger or taller folks. Your hallway and garage door is only 18", I made mine 28 in case I need to move furniture or large items thru there (during my tear-down. I stored my couch, dinette, overheads, and captains chairs in the bus's garage). How tall is your fridge cause 12"x16" will barely hold a six-pack.
Layout - If you're not really needing an oven. You can buy a portable cooktop (1 or 2 eyes) and park it over the refrigerator and do away with the corner.. The microwave can go up on the overhead to regain counter space. Lastly, One thing I did for strength and to save some space is installed my 2x4s sideways so the walls are only 2" wide instead of 4". My wainscots and walls are all Luan for the same reason. Doing this means you have to create your own door frames, but it saved space while maintaining rigidity.
Please don't take this as criticism, I've been where you are and had to redo alot because the space was smaller than I thought. I had to resort to an outdoor shower cause my fridge took its space. I'm a small guy and my 30x30 water closet makes me claustrophobic. I really wanted to have the bathroom door serve double duty and lock off that part of the hallway to make the loo feel bigger, but moving the hinges to the front meant you couldn't get in there from the house section - bad plan. I'll update my build thread this weekend and show you some of the compromises I had to make .