If you only have two weeks for the trip then I would suggest a shorter one - easier on you and the children and for your big destinations make reservations but not for the whole trip. If you pull into an RV park early afternoon you'll get a site. Then you'll have time to relax, have a nice meal or even tour the immediate area a little bit.
If you have all summer then a trip without a lot of reservations is an excellent way to see things along the route that you might not have planned. Sometimes reservations are made and you get there and say "why are we here"? Other times you'll see an area you'll want to explore but can't because you have a reservation further along. Our vote is for 'no reservations' except for the bigger national parks.