I have camped in 49 states and all of the Canadian provinces except Newfoundland and Labrador. We have traveled over 90,000 miles and have visited many national parks. We almost NEVER make reservations. We usually hit the road around 9am and look for a state or federal park to get to by 2 or 3 pm. As long as you arrive before 4 or 5 Sunday through Thursdays you will not need a reservation. If you must arrive on a Friday or Saturday, you'll need to call ahead at least two days for a reservation. This was the case in Yosemite, Yellowstone, Tetons and other popular destinations. The major exception to this is if you are traveling to Florida in the winter. Since public camping and parks are very limited, you'll need to make reservations in advance for each day.
I highly recommend the Good Sam RV Travel Guide. This NY telephone book size directory is essential for making reservations while on the road.
