Originally Posted by vanguy67
Really, how much space do you need to sit on the toilet?
Dude! I get some of my best reading done whilst seated upon the royal ceramic throne!
Comfort is paramount!
In all seriousness, after setting up some cardboard boxes for walls, I've determined that 30" is my personal "comfort zone" for aisle-ways and an absolute minimum for the toilet box. While I'm willing to cut the walk-way down to 24" to move from the front to the rear of the bus, any smaller wouldn't be realistic for carrying in things like backpacks, groceries, laundry, or supplies in general. Don't forget to take these basic considerations in to account, or you'll be cursing as you have to crab-step your stuff inside.
Bathrooms don't have to be all on one side of the bus. Especially if you have a composting, or portable toilet. One of my bathroom designs puts the toilet on the passenger side, with the shower directly opposite on the driver's side. Sliding doors would close off the aisle to provide privacy without taking up too much room. When the doors are slid into their pockets, anyone can walk the length of the bus. With the doors closed, the living space (kitchen, sofa & entertainment center) and the personal space (bathroom, bed & clothes closets) are separated.
Take a couple of minutes to create a mock-up of walls at different widths. Use spare sheets of plywood set next to your house, or (like me) set up a bunch of boxes next to a wall, then try to walk through once with empty hands, then with a laundry basket, then with both hands full of bags of groceries, etc. Basically, think of anything you may carry onto your bus, then try to get it through the opening. If it doesn't work well in the mockup, you'll definitely not want to deal with it on the bus.