If you're using trailer in full sun, summer, southwest-I also recommend the 15K unit. Sure it will be oversized when you're parked in the shade, at night etc. but it is the lesser of two evils. I have a 25' w/ 11,000 btu (lots of experience in hot climates with trailer). If I were spec. a replacement, it would be the 15K.
Added observation-what really would be ideal would be two 8-9K btu units. On hot days in full sun both units could run. At night, on warm days w/shade the load could be handled with one unit.