My 13.5 works flawlessly in all conditions including the mid 90’s and above into the 3 digit heat… if most of my camping was in those high temps I would get the 15… but I can say with confidence to big is not better or more comfortable than to small…
In my 20 trailer the 13.5 was very uncomfortable… it was like sitting in a wind tunnel and cooled the temps very quickly, to quickly… but wouldn’t run long enough to remove the humidity… the cool camper with the high humidity was never comfortable and the cooler you made it the worse it got…