We chose a "shell-on" technique...which can be a pain but allows you to leave the rivets attached to the channel. With the older models this method can be more difficult because the channel is different and so is the method they attached the underbelly. Anywho, I decided to take my circular saw and cut the perimeter leaving about 2" of wood still under the channel. Then, I began the very difficult task of prying the plywood out piece by piece. Each section of plywood butts up to the next and a long bolt is driven through the seam and into the steel cross members underneath, then nutted & bent. You can easily cut the top off of the bolts with a angle grinder but only AFTER you have pulled the subfloor.
Under the subfloor, you see, is MORE nasty fiberglass insulation which we DESPISE, especially now. They're also squirrel habitats...we found lots of nuts, bones, and poop. :( I didn't mention it but we found that as well behind the wall panels. It was a mess.
The large fresh water tank you'll see in the last pic sits on top of a 3/4" piece of plywood that slides in between 2 L-brackets attached to the frame. Then, the tank is split between a smaller support cross member. Basically, you HAVE to slide the plywood out of the the rusty L-brackets with the weight of the tank on it. You COULD set your circular saw to the plywood depth and cut it out in three pieces but we thought we might use the plywood. That took everything in me to pull out...during the hottest part of the summer!
There aren't pics for the edge of plywood under the channel but it's THIS SIMPLE: take your angle grinder with a cut-off wheel/grinding wheel, a Dremel with a cut-off wheel, a hammer, a thick flat-head screwdriver, and some vice-grips and go to town. :) I know that sounds obscure but it really is that easy? You have to work, and sweat, and cuss. And when you get to that point that you wanna give in to the elevator bolts, go work on another task and come back to it later with a clear head. That's how I did it and I got it done. Persistence...
The other step is to get under the Airstream, drill out the rivets and remove the belly pan (the aluminum skin underneath the frame). It's easy except the rivets weren't properly shaved so trying to get my bit centered and steady was a pain and my bit DID eat through some sections of the belly pan.