I've never done an RV bath renovation, but have done several in homes and I'm very familiar with RV systems/plumbing. I also plan on doing mine this summer.
The first thing I would do is measure all dementions in the bathroom. I would get a computer home or room design program...Im sure there are free ones out there.
Once you have the basic room dementions punched in, you can start designing the room. You will be able to see what will fit where. If you want a smaller shower you have picked out, you enter the dementions and you'll know exactly how much room is left for say a larger vanity. I would start with the most important change first. You can of course do this with pencil and paper.
Now the caveats. If you're going to be moving showers, sinks and toilets, you will be faced with plumbing problems. This will mean you will most likely be taking up the subfloor to see if you can do what you want, or what it is going to take.
Also depending on the extent of your changes, you may have to move light fixtures, switches, etc. This will most likely mean you will have to strip the walls to re-run the wires.
If you are handy, none of this should be a big problem. I can't stress enough you need a good, solid workable plan before starting a project like this.
If I were dead set on a complete resto, I would strip the walls and floors, see what I got and go from there.
John & Clare - Buck & Zeus - Jack & Husky
2008 Fleetwood Bounder 38P
Towd: 2 HD Streetglides