Welcome to the world of RV and Airstreams..
ALL new members should take time to practice and develop "Search" skills using tab above, to find hundreds or thousands of messages already on the system covering questions you have..
If you clicked and typed in "air conditioner replace" or maybe added carrier, you'd find stories, pictures, etc...
To be kind, the general rule is all of them fit a 14" square opening in the roof. Airstream used a plastic tray with a drain fitting to a tube down the side of the trailer between inner and outer skin to handle condensate water. Not all brands of A/C's will fit this tray, as it was intended for Dometic. The Carrier's use a "slinger" system to spray condensate onto warm coils to prevent dripping water and enhance cooling. Others may weigh in with recent experience and do-it-yourself experience..
Depending on climate, some 13.5K BTU units might not keep larger trailer cool enough, and 15K BTU might be better option.. You also have choice to include electric heating rod to spare furnace on cool nights, though furnace should be used if temsp below freezing, since furnace warms "basement" under floor and keeps plumbing and tanks from freezing...
In Theory, there's no difference between Theory and Practice, but in Practice, there is usually a difference...