You live in Miami so you know what FL July heat is like. Perhaps you are young and won't be as bothered by the heat & humidity as some of us. Perhaps you plan to spend all your time in the parks and pools. Assuming all this is true, go and have a blast!
Your idea about bringing a sleeping bag & air mattress is good. Rather than a sleeping bag, you may just want to bring a light blanket & sheet - you'd end up sleeping on top of a sleeping bag in the heat. Also, bring:
your clothes, with extra changes
cool nightshirts & baggy shorts for sleeping
two swimsuits
toiletries, sunscreen, bug repellent
bandaids or moleskin for preventing/treating foot blisters
money and more money

your own pillow, if you need one to sleep comfortably
flashlight with fresh batteries
couple pair comfortable summer shoes
personal water bottle to bring in the parks
ziploc bags of laundry detergent & dryer sheets (?)
a fan & indoor/outdoor extension cord, if possible
Ask your friends with the tent whether they will have all this... with the tarps needed and a cooler or two. When you get too hot & sticky, get inside a pavillion, an indoor attraction or video game area, a restaurant for a cool drink or go swimming. Disney Quest is all indoor. So is the movie theater at DD. Avoid long outdoor lines midday. In July, you can't worry about seeing all the attractions. The lines are too long and too hot. Hope you have FUN with your friends!
