For a long trip, we leave home at either 5am or 6am and drive till about 3pm. Usually that gets us off the road early enough to find something along the route. Calling about 30 minutes ahead of where you are seems to work well also. Agree with the use of RVParkReviews.com.
Anyone remember what it was like in the dark ages .... before we had live internet in our motorhomes as we barrel down the highway at 60+ mph? And haven't we always had GPS devices in our vehicles that will route us within a few inches of where we need to be? Amazing how times change! Both the internet and GPS make it a lot easier to find those ad-hoc stopover parks while in unfamiliar territory!