There'd probably be people effectively living in the park in their RV if entrance was free. The entrance requirements also help enforce that rangers inform visitors about dangers, closures etc.. upon arrival.
If you’re quiet and out of the way, I dare say you could live on BLM land permanently and nobody would know, let alone care.
And some BLM land is really impressive.
This mostly applies to the west coast though. The eastern US has very little public land that is freely accessible.
In the wild parts of the US, even the government tends to adopt the “live and let live” ethos of that type of country.
An annual park pass isn’t that expensive. People who live in an RV would spend far, far more driving to and from the park for various things they need (gas, supplies) than they’d save on the park pass.
Regardless, having a pass doesn’t eliminate the rules regarding duration of stay. You can’t live in a national park even if you have a pass. If you try to do it in an RV, you’d get noticed and cited.
https://www.wired.com/story/meet-camperforce-amazons-nomadic...