As a full-timer you will have endless time so definitely don't make reservations. We rarely did unless we 'needed' to be somewhere (visiting family, etc.) There are always campsites to be had especially if you pull in early afternoon. For weekends we would also be at a destination by Wed or Thurs. Don't try to move around on a weekend. For the major 3-day holidays, we stayed away from any kind of water: pools, lakes, rivers. That's where the weekend warriors go so we let them have their fun and we would move to those places when they went back to work.
Even for the major western national parks we got in without reservations. Some of the campgrounds don't even accept reservations and that's where we headed early morning when folks are leaving. We stayed nearby the previous night.
Don't try to rush your travels like having a destination from Missouri to California within a couple months time. Concentrate on a particular state or a few states and cover them thoroughly before moving on.
Stay off the interstates unless there's no other choice. The secondary roads are just fine and you'll see more of this beautiful country.
Purchase the 'Mountain Directory for Truckers and RVers'. It comes in Western states and Eastern states. We consulted it constantly as we liked to explore those mountain roads.
It's great lifestyle. Have fun!