We had a 31 foot Springdale TT that had one single 13,500 AC (ducted air). The camper was open, no walls, except for the bathroom, from end to end. We traveled from Florida to Minnesota and never had problems with the AC not able to keep things cool enough to be comfortable.
When we traded and got the Outback 35 foot TT with walled bed room and separate walled bathroom, it came with a 13, 500 AC ducted air also. When outside temps were below 90 and we could start the air in the morning, it did well. Over 90 or trying to cool it down after traveling all day, it just couldn't keep up.
Last Summer we swapped over for a 15,000. Same TT, same weather conditions, and a BIG difference! Over 90 and we chilling inside now. Best choice we could have done.
Considering you are in a climate prone for longer hotter days than us Northerners are... well, let me just say; I know you will NOT be disappointed if you get the 15,000. But you might be if you settle for the 13,500. Considering where you live, I'd settle on only the 15,000 after my personal experiences now.
Edit: I'm experiencing finger "fart" today. I can't type numbers today worth squat! ARRR!.