I created a 3D model of our bus in Autocad but, honestly, it was good for planning purposes but not much else. Good for getting a rough idea of the layout. It was nice to have a 3d walkthrough and to insert stock furniture and appliances. I pretty much just used rough dimensions for the model I created. The textures, wood grain, fabrics, etc. were just never quite right when I rendered the model.
Once we started the actual building of it, those plans went out the window and the interior grew organically as the process played out. The appliances, cabinetry and furnishings were much different in the real world than they were in Autocad. We altered our layout on the fly when we had actual pieces to move around.
I'd recommend getting as close as you can, dimensionally, in Autocad and roughing out the design with the Autodesk Architecture block for furnishings/appliances/cabinetry. The work the details out when you have a bus and the objects that are going to go in it. Also, I'd recommend not buying too much things ahead of time....or if you do, save the receipts so you can return them!