If you start off placing your closets/cabinets over the wheel wells, it will save you a lot of hassle. Our wheel wheels are about 48". Absolute minimum PER PERSON for hanging clothes closet space is 18", 24" if you need to carry a heavy coat. That's enough clothes for 8 days (7 sets dirty and 1 set clean to wear to the laundromat).
For fulltiming, the ability to close the bedroom/bathroom area off from the rest of the bus is a benefit. That way you can only heat the smaller bedroom/bathroom while you take a shower, get ready to go to bed or get dressed for the day.
About all-in-one toilet/showers... Your metal parts in the toilet WILL rust because they will stay wet for so long and so often. Replacing some of the parts will result in a bill that will make you decide that it is the same or cheaper to replace the whole toilet.
We have the same shower pan that is linked to above (only we bought used and it cost us $25). This is a thick pan that we set flat on the floor. If you decide to install a toilet in the pan, I would suggest that you tuck it into a corner at a 45° angle. Use PVC "boards" that you can glue together with clear PVC glue to make a raised deck (sloped ever so slightly towards the shower pan center/drain for water drainage). We used a kitchen sink drain for our shower pan drain. It's nice to be able to drop the stopper in the drain to catch hair (I shed a lot). We also tend to use the shower stall as a place to rinse out those big rubbermaid storage totes, etc. Using the drain stopper to hold water in the shower (only a few inches, allows me to soak my feet when needed. They hurt from standing on the concrete floors so much at work.
Kitchen sink drain in the shower pan. Fit perfectly.