The pictures you posted don't look too bad for rust. A Florida trailer or anywhere else where the climate is warm and moist might cause steel to rust faster than in Phoenix or Bakersfield. It seems like the 70s Airstreams (and Argosys) are more prone to rust problems than other vintages. A 24 footer (Argosy 8 meter?) is a nice size as it came with dual axles and yet still rather light weight. This put less loads on the frame members.
You can also assess the frame rust through the rear bumper storage compartment and the fold up step box under the trailer. Rear end rainwater leaks are tough on the frame as well as the subfloor. Many Airstreams leak here. Inspect the rear of the trailer carefully.
Also carefully probe the subfloor around the interior perimeter of the trailer with an awl or ice pick. Look for spongy plywood. This is likely below windows, the entry door, and the rear bath area. Rotted plywood can be replaced, it just takes a bunch of work.
I think the most important thing in any vintage Airstream purchase is the body, or shell. The body is expensive to repair. The rest of the trailer is very repairable given enough time and money.
So the shell, subfloor, and frame make the semi monocoque construction that makes Airstreams a bit like old airplanes. Strong and light. But all three members must be in good condition for it all to work as designed.
Here is a photo of a rear frame member that has rusted holes in it. It is obviously lost strength. And here is a photo of my 66 Trade Wind rear end frame rails "as found". Rusty, but I judged the majority of the steel's strength is still there. And here is a photo of a mid seventies Argosy that's been restored. I didn't realize Airstream used the Argosy brand as a field test bed for new design concepts. It has steel end caps (thus the full body paint) and the new "panoramic" windows in front. Now all Airstreams have panoramic windows (at least most do.)
Hope this helps you...