We have an all in one bathroom. 3' x 10'. Shower is a 3x3 pan on one end of the room. Toilet is in the middle and full size vanity/sink on the opposite wall. All on the drivers side about mid ship. Bathroom door (swings out into the hallway) is basically in the middle of the 10' wall with plenty of room to close the door behind you. Shower is a little cramped in the head room department mainly because I'm 6 foot and the shower is up against an exterior curved wall. I wanted to make sure the bathroom was big enough for either the wife or I to take care of business without to much issue. With two of us living full time in the rig, we need things to work properly and to not be a burden. Living in a small space causes all sorts of sacrifices and one area I did not want to sacrifice to much was the bathroom. (gotta keep the misses happy)
Shower valve is on the same wall as the kitchen sink and water heater so that made for some really short pex plumbing lines. We also used mainly all home depot style fixtures and such. Basically in hopes that when something breaks I can fix it easily with household items and not expensive sub par rv style fixtures. Sounds good in theory at least.
Another item to think about is dumping the gray and black tanks. I kept all of our tanks on the drivers side in hopes that it would make using dump stations or hook ups at an rv park easier. So far it has worked out well but I do notice quite a few dump stations that would allow you to enter backwards and dump from the passenger side.
All just food for thought. No right or wrong way to go about it. Have fun though!