2nd a/c

I have a 35' Montana with a 15k a/c about center of trailer. I want to install a a/c in front bedroom, which is factory prepped for it. Just a non ducted unit. What size btu should I be looking at? I live in south Louisiana, so it's hot. I was leaning towards 13.5 but don't know if that's to much. I also want it to be low profile.

Thanks