Welcome to the forum and to RV fun times. Some great RV destinations for escaping heat with plenty of camping opportunities are the Oregon Coast including state parks (we were there in early June), the Canadian Rockies, and some U.S.National Parks like Yellowstone, Grand Tetons, Yosemite, Redwoods, Bryce Canyon, Glacier, and Rocky Mountain. Temperatures vary of course.
On the East Coast, there's Acadia NP, Maine coast, Atlantic Canada...many more all over, especially higher elevations.
Some parks and destinations shut down in winter, some are great in the spring and fall, some like Crater Lake are only up and running for a short summer season. Just takes a little research.
Winter escapes from the cold are a whole different list. Enjoy!