When talking about 2024 T3, you are talking about an alloy and a temper. "Alclad" is used to describe the layer of pure aluminum that you might find on a piece of 2024 T3. Judging from the age of your trailer, you probably needn't worry...
In the early 70's, Airstream started experimenting with different alloys, specifically 6061 T6. In the later 70's, they were using 3004-H18 panels.
I would recommend the 6061-T6. I replaced my entire belly pan and several exterior panels with it on my '73. I used .032, which is what the factory used in those days, but I used .040 in "high stress" areas like the rear panel and and a huge patch on the roof where the old AC was taken off. If you are going to paint it anyway, you definitely don't need 2024 T3 Alclad.
I am currently re skinning my entire interior with 5054 in a .040. This stuff is pretty soft, and I sometimes have my doubts about whether I shouldn't have gone with the 6061 here as well, but I've gone too far to turn back now. I chose the .040 with the expectation that I will see less rippling in my walls with the thicker material. The original skins were .032.