Not exactly what you want to know, but simply knock a coule feet off the total length for the bumper and hitch and figure 8' feet for the width.
About half of the interior space will be used up with cabinets, beds, and seating.
I don't know that I have ever seen square footage listed as a spec.
Jeff & Cindy
Remi & Hunter
'09 27FB Flying Cloud
'91 350 LE MH