If the trailer wasn't sold with an air conditioner on it, the company didn't reinforce the roof. No way to retro-fit. There were some trailers converted with a support frame added. One way was to put the frame on the inside and the other way was to put the frame outside. All of which adds weight
to the top of the trailer. Pictures were here, I don't know if they still are. General description of it is: A long anchor brace along both sides of the raised area of the roof. Two vertical and then horizontal braces on each side attached to a load carrying square frame to mount the a/c to. I never heard how that worked out in the long run.
Functional, but there are other ways to install a/c.