Where to meet family between Colorado and North Carolina?

Another vote for northern Arkansas. Hot Springs, Crystal Bridges, Eureka Springs have been mentioned (coming from CO you'll like the setting of Eureka Springs). There's also Pea Ridge NMP, Blanchard Springs Caverns (one of the most pristine caves around), canoeing on the Buffalo River.

Not a "higher end" RV park, but check out Lake Ouachita SP. FHU and the kids can swim in the lake. There are also cabins there if any of your group doesn't want to camp.