It's possible to do a complete floor replacement with the shell on, but that's the hard way. Full Monty is more intimidating, but easier to do.
Dropping the belly pan involves drilling out a ton of rivets. Some are hidden, so pack your patience. You'll need to remove anything attached to the belly pan like propane lines and stabilizer jacks. You will get covered in all manner of disgusting stuff, so use safety glasses and a dust mask.
I strongly recommend you spend some time camping in your Airstream before you start disassembling it, if at all possible. Camping will give you a much better idea of what your priorities are, what you want to change, and what you want to keep. It's much easier to preserve something than it is to rebuild it when you've carelessly ripped it apart.
Dropping the belly pan is opening Pandora's box. You will find more stuff you want to fix, and depending on how much you find and how much time you can spend on repairs, it could be a year or more before it's camp ready again.