One of the first conversions I ever got a tour of belongs to a friend of mine. He did a fantastic job on it. But he told me he had one regret, and that was that he made the shower stall too small. He was ok with it, because as he said, you're only in there a few minutes, but I could tell it bothered him because he took a lot of pride in his bus and it's design, and this was obviously a "failure" to him.
That conversation has always stuck in my mind, so when I planned mine, the shower was an issue. I had limited space, but needed to make sure it was adequately sized. So, I measured the one in our master bath in the house. It's about the smallest fiberglass unit made, but is still comfortable to use. I modeled my bus shower after that, with some slight modifications, trying to preserve the basic measurements of L x W, and I could not ask for a better result. It's the perfect size for use, and uses no more space in the bus than was necessary. I used a curtain, instead of a door, which gives the feel of more space in the bedroom when it's open, plus keeping it open allows the shower to dry out when not in use. Right now, the shower is the only "jewel" in my conversion.
I did the same for the toilet. I measured the ones in the house to get an idea of minimum clearances needed to the sides and front. I'm not done with the bathroom yet, but have the side walls up, and am very happy with the clearance there, so far.
I gave up on trying to use cad to do the design. Would have been different if I'd had more experience with CAD, but there are so many other factors you don't think about till you actually get into the building stage.
The cardboard and tape approach is probably the best, though. Build out your interior in refrigerator boxes and duct tape, and then take a weekend or longer trip. Adjust things as you go and when you have something that works for you, copy it.
Whatever you do, make sure you use the coach while you're building it. That's when you find out where you've made mistakes in the design, and the sooner you find this out, the easier it will be to correct. I have several things I need to change, already, and had I waited till it was all done before using the coach, I'd be very unhappy.
craig