Only at certain times. If you travel full time, you can find times where even the most popular national parks aren't crowded at all.

Yes those times are called "Winter" and they aren't fun to camp during. Summer is the best time always, and summer in any of the popular national parks is miserable.

National parks in Spring and Fall can be great and aren't that crowded during the week. E.g. anything between Easter and Memorial Day tends to be nice weather, most roads are open and visitor numbers are low.