Congratulations on getting your Airstream.
After you are sure it isn't leaking anymore, I think you can just get rid of the rust and fabricate an access panel. Just don't put anymore extra holes in the tank when you attach the new access panel
. I don't think you will have to drop the tank enclosure.
A year or two ago I had to drop the tank enclosure on our '06 to fix a plumbing coupler issue due to my inexperience and stupidity.
I used a hydraulic floor jack to support it until I removed all the bolts and used the jack to lift it back up when I secured it again. When I dropped it the tanks stayed in place, but I don't remember how they were supported.