Welcome to AS Forums! Glad you found this site -- there is much discussion on most any topic if you can come up with good keywords to use in the 'Search' area of Forums. Your job will become surprisingly comprehensive by the time you identify all areas that need work. Buck up there -- you will end up with a unit of so much more character than those who have bought new (with a whole S***load of other issues to contend with...).
Your trailer may have benefitted from the Southwest's dry climate but that also contributed to heat-related deterioration of seals, caulking, etc. Everything about taking an Airstream apart is progressive. How much do you really like that carpeting? If you aren't disassembling significant amounts of your trailer you probably haven't found all issues of floor problems. I assume you are working with something similar to the pics found at http://www.vintageairstream.com/arch...ari/index.html
I have found having an inexpensive digital camera allows me to look back and remember what I did months before in taking something apart. It is likely that a sequence of simple Phillips head screws holds your couch in place. Your horizontal supports will disassemble from front base parts and in a different fashion from the interior skin under the front window. More parts of the assembly will reveal themselves until you have bare inner walls and bare floor (sorry about any new discoveries...). My unit was leaking badly around the front window and resulted in 3-4 square feet of bad floor. Remember to address water infiltration sources for every problem you find.
Do a search on the keywords: rot doctor. This is a thinned epoxy product that penetrates old flooring and can be useful if there is actually any reasonable structure remaining. Things get complicated pretty quick if you have through-and-through destruction. You'll have much to read about. Be very cautious about closed spaces and your respiratory health! Please take note that any treatment of other screwed-in portions of your trailer may be permanently bonded in place if you don't have everything disassembled as much as you can before using Rot Doctor.
Use 'Search' with abandon. Ask a question to revive an old thread if you have questions in that particular area -- or come back to this thread. Come back often -- and then tell us where you are Airstreaming!
Our snow isn't all the way gone but I saw somebody moving their ... looked like a Safari about your vintage ... later on Easter Sunday afternoon, between North Branch and Taylors Falls, MN. Aluminitis is everywhere!