Best wishes to you on your interesting project.... thanks for sharing with us.
I don't have a concrete suggestion.... (or even a wooden or steel one...) but a couple of ideas for you to consider. They're not much but in case it helps...
I converted a two car garage into a granny flat that has a shower, toilet, laundry, bedroom, lounge room and kitchen. I spent weeks trying different floor plans on paper, none of which seemed doable before I finally came up with one I'm proud of.
So my two suggestions are....
- Use graph paper to model the bus as big as you can in outline. Then make scale paper outlines of the bed, lounge, shower etc. Lastly overlay a plastic sheet you can draw walls on with whiteboard marker. This allows you to try different orientations and plans by moving them around in the outline.
I did that multiple times and eventually a layout suggested itself that started to work. I was honestly surprised that a particular layout worked much better than others.
- The other suggestion is use partitions with alternating sides of access to make the place bigger and more private than it would be otherwise. Small half walls etc can actually make the place seem bigger.
I know that you have constraints of heights etc but the earlier suggestion of giving more privacy to loo etc will probably make the place seem bigger and more liveable.
Sorry to type long.... Good luck with your project.
