Depends on the length of trailer. My 15k BTU A/C does a so-so job at cooling down my rig, come the Texas summer, and my rig is about 27 feet from ball to bumper, without slides.
Here in Texas, if the rig is over 24 feet and has slides, it needs a 15K BTU AC. If it is over 30 feet, it needs two A/Cs, preferably in a "H" design so either A/C can use all the ducts.
If someone tells you otherwise, they are fibbing. Texas is not getting any cooler come summer, so the last thing you want to skimp on, is A/C capacity.