Continually looking for inspiration on layout some ideas on layout in small boxes, you may want to take a look at http://www.taylorcoach.com/specifications.php#features
There smallest is 700 lbs and they extensivly use rear entry in up to their 17 foot models. Just shows you can get alot in a box and still keep it lite.
Now if I could keep mine lite, then i'd be gettin somewhere