Making things square is much simpler than making things level, in my opinion. Your idea of building things outside of the bus and then moving them in has been working out well for me so far. Just be mindful of the door width so you can actually squeeze it in there when it's finished.
Edit: I now see that the question was answered and this thread died many months ago, but was dredged up today by probable spammy activity.
Thanks for pointing that out DeMac!