We took a similar 40 day trip in 1995 with our two children, then 11 & 16, in a 28 ft Class C. It went perfectly. We stayed in campgrounds with pools each night so the kids could get some exercise. They especially enjoyed the theme parks, so don't pass those up.
Since we started from Washington State, we drove across the US to Niagara Falls, then New York, Philadelphia, Gettysburg, Washington, Kentucky (Grand Ole Opry), then back west across the US to Los Vegas, Grand Canyon, and up the coast back to Washington State. We stopped at every tourist trap and scenic outlook we saw, as well as National Monuments, Parks, etc. We didn't see "everything", but we didn't miss a whole lot either.
