We were there this past July (5-9).
It will be hot. It will be humid. It will be PACKED.
But, it is manageable. Take breaks, bring mister fans, keep hydrated, use fastpass. Best advice I can offer is to follow a touring plan. We followed plans from Touring Plans and even though it was oh my goodness crowded, we got everything we planned on done, and the waits were not as bad as they could have been, since we utilized the plans.
I just got back from a second trip w/my mom and DS at the end of January, and I can tell you that it was WAY more enjoyable than July (LOL) but with my DH, his vacation is the first 2 weeks of July, so that's when we can go. I think that if you can get yourself psyched up for the fact that yeah, it's going to be hot, but make a concerted effort to not let it get to you, you'll be fine.
ETA: To answer your "how hot" question..most days, when we got in the car, the outside temp was already reading in the 90's. OUCH.