I thought about doing it, but figured I had enough regular storage. A hidden gun safe was one consideration. However, one thing I did do though when I was underneath was install several "T" nuts in strategic places under the twin beds. This allowed me to run the battery strap and also to screw down my fresh water tank. I have a couple extra in case I needed to strap something else down. I thought about some boat rings too for strapping down a bike or kayak inside, but we're going with more of a museum restoration, and I didn't want anything visible like that. It is nice though about the heavy battery and full water tank. I won't have to worry about a big bump in the road pulling out the little screws they originally used. I would consider a a sealed flashed pan that drops into the space.
The only issue would be near the outside walls where the frame gets strength from the plywood. See if you can add a strip of steel that bridges or frames in your cuttout from below if possible.
the other thing I did was back in the back where i had a rear tank, there was a missing cross piece compared to the rest of the trailer. Meaning, the tank took the place of one of the side to side frame members. As a result, standing in front of the toilet, the floor flexed a little. So, I got a section of steel "L" channel, and bolted it from below. That was the equivalent of moving my missing steel support to the top instead of the bottom. It's right behind the rear bench. So, a little bit better than Airstream's original design. Not sure what you have in your model. I also walked along the floor and used self tapping screws for any soft spots or squeeks. Just used a forstener bit, recess a tiny, put in the screw, then bondo over it. Made my floor more stable.
Just some things to think about while you have the floor available.