There is a very nice resort called Indigo Bluffs near the Sleeping Bear Dunes National Lakeshore in Michigan. It has a regular campground side that is very nice as it is, but they also have a "luxury" side which we found absolutely fantastic. We stayed on the "regular" side, but had we known how nice the "luxury" side was, we would have happily paid the higher price for that.
The other nice resort that we have been to is the West Glacier KOA near the west entrance to Glacier National Park.
Those are probably the two nicest places we have stayed.