5150 may have potential. Based on their gooseneck trailers and motorhome interiors they may be able/willing to do a custom living quarter gooseneck.
http://www.5150racetrailers.com/
Another option would be the custom coach builder in my neighborhood, Marathon. I've seen big tag trailers at their site and their interior work is pretty good. Possibly sub the interior out to them?
http://www.marathoncoach.com/index.cfm