Brisan0623,
Like everyone else has pointed out, the 15k would be the safer bet. I had a 31 foot travel trailer with a 15k A/C and you could hang meat in it in North Carolina, but I like it cool. Whether the 13k will work for you is largely a function of your preferences of temperature and humidity, and how, when, and where you use your new coach. I know that Indiana can get quite hot and humid at times.
If you take some steps, you can maximize the ability for your equipment to keep up. You should take a look at the Heating & Cooling Users guide. It can be found under the Tools tab on this website.
Certainly, if you have any opportunity to upgrade, right now is probably your best shot. For peace of mind I would attempt to negotiate that with your dealer before delivery.