Private mail forwarding companies are not part of the USPS. You can set up a private mailbox and then call or email to make arrangements for forwarding that mail to another address (such as general delivery at a post office). You can also make arrangements with the post office to have your mail forwarded to the private mailbox. You can count on the reliability of the private companies. They do this for a living. You cannot count on the reliability of the USPS. I made numerous visits and calls trying to make sure my mail was forwarded but that only happened in a haphazard fashion.
Many of the mail forwarding companies are located in Texas, Florida or South Dakota. Due to tax and other considerations many full time RVers establish residency in one of those States and also use that State to include their mailing address.