I’m converting a 5 window Thomas right now. It’s kind of coming together like you want yours. I’m in the early stages of the build and so far the challenge has been finding space for the holding tanks and water heater. I finally managed a 20 gallon black tank and a 28 gallon grey tank. Not quite as much capacity as I hoped.
Not to disparage the short bus, I think it’s a good choice for what I’m doing and it sounds like for what you’re doing. Just be aware the under body space limitations.
If you spend some time looking st short bus conversions you’ll see that the head and shower usually take a minimal approach at best. Spend some time examining this aspect, especially the head.
If you haven’t seen this one, here’s a video of shorties I like. Lots of good ideas in there. But take note of the minimalistic plumbing.