Rooftop airconditioners are renown for having problematic input/output airflow issues. Check out this quick youtube for ideas on improving your efficiency: https://www.youtube.com/watch?v=rxCljBVi8DU
Here is another link from a Winnebago Forum.
A quote from the forum: QUOTE" We have a NewAir portable ac. We only have one AC and it was very loud. There’s a new accessory you can buy to quiet it down and enhance performance but it’s a bit expensive in my opinion we made one ourselves. There are some YouTube video about it that might help. There’s also a great write up here." ENDQUOTE.