I agree with Bobcat that northern states would be best in summer for a rally. I love The Rocky Mountains. But beware, just so you know it can get toasty there as well. In mid June, a few years back, we were in Hardin Montana for Reenactment Days. The temp was 105. But then again the next night at a higher elevation in West Yellowstone it was 27. So you never know what you are going to get.
One other thing worth mentioning is that I searched for larger RV parks in the areas mentioned and could not find one large enough, or one with a convention center to accommodate our size requirement.
Peace
Dave