If you're from out West (or anywhere where the humidity is usually lower) it'll definitely be an eye opener. Not to start the old "it's a dry heat" vs humidity debate, but in my opinion humidity DEFINITELY makes for a much more miserable experience even if the temperatures don't get into triple digits, especially if it's not windy and it's stagnant. The worst thing about the humidity is when you sweat, the air is so saturated that your sweat doesn't evaporate quickly (like it does in drier climates.) As a result, you can pretty much plan on being permanately sticky. Fans help a little, but don't expect any miracles as it just cools the sweat that rapidly builds up on your skin.
We used to stay in a pop-up camper alot during the summer. Our first pop-up didn't have A/C and I don't think I'd like to repeat that experience ever again. I think I averaged 3 showers a day and I still felt funky (and I'm a Southerner and used to the humidity.) I'd personally plan on spending most of your time at the campground in the morning and in the evening when it's cooler. Then go to the parks during the hot part of the day. Since most experienced Disney park goers usually use the opposite strategy (hit the parks early and then come back and crash for an afternoon nap before heading out again in the evening) you might actually face slightly thinner crowds as a result.