A picture of what you are talking about would be a great help.
But, If your AC is inside an enclosure that contains the rest of the trailer, understand that a similar amount of heat will be generated in the roof enclosure as compared to the cool air being placed inside the trailer.
So long as you can give that warmer air a means of escape, with outside ambient air able to replace it you should be fine.
Also understand that the rooftop AC will generate condensate (water) which will drain toward the sides or whatever low point is available, just like you see drain from a window AC. You must give that somewhere to go.
