It's been many years since this thread started, but it's a valid question and came up first on the search, so I figured I'd add.
+1 for MagicPlan. I used it to map out the inside of my bus conversion. That only counts as one plan, so it's basically a $2.99 app and for that, totally worth it.
I'm able to export the resulting floor plan in a variety of formats (images, pdf, svg, obj).
What I'm working with now is I built the floor plan with MagicPlan and exported it as a JPG which I can use as a template in the desktop app
Sweet Home 3D (which is free, but you can buy furniture packs - [url=http://www.sweethome3d.com/]Sweet Home 3D). So far, works really well.
Here's a pic of the 3d model it spit out of my short shuttle bus with the accordion door, rear doors, and even the side windows in the right places (yes, it captured elevation. Though I just blocked out the dashboard rather than make it
really accurate).