I've been playing with FloorPlanner.com for awhile now. It's pretty good for overhead visuals to monkey with. Best part is that its free and on the web so you dont have to worry about loosing it or making copies to send or whatnot.
I started with just a basic idea of how big a 40' bus is and all of my plans are on a single sheet and use the same dimensions. When I finally can afford a bus, I can make the measurements accurate and finalize a set of plans. But for the mean time, nothing is set in stone, so I set some sizes that I know, and just adjust others to look about right.