It sounds like you are looking at new only?
Somerset has the next evolution of the Fleetwood Niagara, 14 ft box high wall -
http://somersetrv.com/campers/NiagaraFor used there are a few older models that are quite large. Jayco had a tandem axle unit that was either 16' or 14' but I can't find it at the moment.
There's also the Coleman Avalon with a 16 ft box, 28 ft, 10 in. when opened -
Quick SpecsMake sure you get the year(s) that it is as shown above.
The Rockwood HW296/Flagstaff HW29SC seems to be the reigning champion for size for new models, at 27 ft. 9 in. opened.