Last summer we planned to drive for 5 hours and 300-350 miles each day but we drove about 65 mph on the highway, maybe even 70 when the speed limit was 75.
I used the Good Sam trip planner because it marks the campgrounds along the way. We made reservations on the way out because we had kids with us and didn't want to be stranded without a camp site when we arrived.
I thought we could fit in some sightseeing with that much driving but I was wrong. We were too tired of driving and just wanted to stop and rest. The days were not too hard on the kids though--they were good travelers. (5 and 3 years old).
We traveled from Denver to Indianapolis/Chicago and back to Denver. Three days out and two back--figured if we didn't have energy to do anything when we stopped we could at least drive further each day on the way back. We didn't have a reservation mid-week on the way back but it was not a holiday week.
Maybe you will decide whether you want to get where you are going quickly or if you want to sight see along the way. Have to have more energy than we did to do both!