Our tactic is to drive until we feel like stopping, an hour or so before then we call ahead to reserve a spot at a commercial CG. This rarely fails with the exception of major holidays. The 4th of July for example. You need a good CG directory to pull this off and we usually
use this Good Sams one.If you use boondocking sites it will be first come first served most of the time.
There are online directories but they just don't have half the CG that are in GS book.
Using the above method we had to call a second CG once during a 6 week long crosscountry trip. We did have reservations for inside National Parks tho.
If a CG charges more than 30-40 dollars we often will move on. Most of the non-chain CG will be in that range. This depends on your location of course, the Florida Keys will be expensive for example, Delta Utah will not be.