I completely understand doing as-built never works out as planned.
I worked as a draftsman for architects and engineering companies for quite a few years.
My reason for doing the design prior to purchasing the bus is because I like to work out out all the little details of why I would certain things in an area such as how the kitchen is layed out, sink locations, burners, cabinets..
I want all these details worked out prior to getting the bus and feeling the need to rush through and not have a well thought out plan. Of course after I got the bus I would measure everything make a 3D model and construction documents.
I would greatly appreciate some DWG/ DXF files.
Here's a link
I came across that has dimensions/ guidelines.
I also sent an email to Bluebird requesting any cab files that would be available, I doubt I'll hear back from them.