hi philip and welcome to the forums...
that's 3 adults and 2 children in a 28 footer?
tight, very tight.
is it gonna be parked or moving much?
adding the walls and other bits, for another enclosed space midship, will add significant weight to the unit...
yes there are folks who can do this work
but it might be easier just to buy the ONLY recent unit that comes with 2 enclosed/separate sleeping areas...
and really does have space potential for your possy...
the safari bunkhouse 30 footer...
just happens to be one up 4sale on ebay right now....
eBay Motors: 2005 Used 30 ft Safari Bunkhouse Airstream Trailer RV (item 120160862949 end time Sep-21-07 10:21:24 PDT)
and since it is in ohio, you could take it back to the factory for any additional tweaks needed.
cheers
2air'