Norm & Rich,
While it is true that I have replaced two full sheets (4x8) of flooring, it was done in two different trailers and was the rear most floor section.
I think the problem with full sheet replacement throughout without removing the shell is that there is a C-channel that starts just forward of the last sheet that the plywood fits into.
It is certainly possible to create enough separation between the frame and shell at the ends but I'm not sure how well you could do it temporarily in the middle. (My mind is racing though, as I think of how this might be accomplished.)
I don't blame anyone for not wanting to do a shell-off when you consider all of the disassembly required. (Actually disassembly is kind of fun, it's the reassembly that gets me.) (Of course, to replace any section of the floor, you've got to disassemble/remove cabintry and appliances.) I'm just wondering if you could jack up the shell 2" in sections (rear, middle, forward), slide a perfectly cut piece of plywood (whatever the exact width should be, maybe 96", I'm not sure) in from one side and gently pull out on the sidewall to allow the plywood to fit into the C-channel.
I'm not sure this would be a factory endorsed method.
Another option would be to try to slide each piece in from the back but you'd probably have a heck of a time sliding a 4x8 piece of plywood 12' with channel on both sides. (You'd have to jack up shell in this situation also.)
Toaster's idea is probably a lot easier and if you cut your seam in the middle with a 45 deg angle (for that matter, maybe even angle your direction, an engineer could evaluate that one for us) and put a "bandaid" piece underneath, glued and screwed.
I do have some rotten floor to replace up front in the Sovereign
so when I get to it, I may try a few different techniques (or maybe not).