Try Hawk Engineering in Jackson MO. He has built a few "unfinished" units that I know of.
We just completed a 42ft coach with an 8ft garage on a KW T2000. We used a 2001 model year truck, stretched frame and build a new 27 ft box with lower garage unit just like you mentioned.
Price was very reasonable and the build quality is excellent. But more importantly we got exactly what we wanted, the way we wanted it. We have built a few trucks before. We knew how we wanted this one to work so it was important to us to get it right the first time. You might be suprised at how inexpensively you can build a quality coach on a used truck in the middle of the country...
Jason does "all of the work" at his shop. They have metal shop and cabinet shop on site. It is a great facility complete with a paint booth big enough to handle a 45ft coach. When you see the facility (located on his property) you will understand the quality of the build.
His website is not that flashy but he is a coach builder, not a bit-bug.
Ask to speak to Jason and tell him that Radman from Kirkwood recommended him.
Let me know if you have questions about our experience, we are happy to help.