The majority of USFS and BLM campgrounds have campsites are on a first come basis. In the national parks and at private parks reservations are more likely to be needed but it is much less of a problem with a truck camper.
Often we find spaces that work well for us but would be too small or impossible to back into with with a tow vehicle pulling a trailer or a motorhome longer than 28 feet. We can fit into any campsite and that makes a world of difference.
Even though Yosemite may fill up at the valley campgrounds there are more than a hundred places outside of the park where you can camp. The USFS maps are great for finding these places as is the book by Tom Steinstra.
We have only needed a reservation for the types of places we try to avoid that are crowded and noisy and for us a last resort.
