Welcome to the forum! :wave
I don't have the exact answer to your question, but you came to the right place. We have all shapes and size of molded fiberglass trailers and someone will know.
In the meantime, perhaps some general information will help. I have an older Casita
that has an air conditioner in the back window. I have looked into what all is available, because it's old and I know it's going to be going out soon. It's a basic, small window air conditioner and I'm guessing yours probably is also. As far as efficiency, I think any of the newer models would be more efficient than these 'ancients' that we have...but I don't know that for sure. Somewhere I read that a unit that has a rotary compressor is suppose to be better for our trailers, because they pull less power on start up. Don't know that for certain, just something I remember reading somewhere.
If what you have is just a regular window A/C then Walmart has one (Fedders) on line for $98 (5,200 BTU). Since mine is in the back window, weight
is extremely important to me and this one meets my needs. It might not be a concern for you being down below.
First, find out what you have...then you will know what you can use. I have no experience with the Fedders unit, it just seems to come the closet to what I need.