Thanks to the Internet, email, online banking, and online tax filing, we are closer to a paperless world than ever before though it will never be 100% paperless. Snail mail will continue to exist if only for packages and junk mail. Like it or not, you still need a way for snail mail to get to you. For RVers, just a post office box or a private mail box will not do because you need the mail to get to where you are now, not where you will be in 6 months. If you are a full-time RVer, you also need an address to establish residency.
You can use family and friends for mail forwarding and even an address but remember they have lots of other things going on in their lives and some of those things, believe it or not, are more important than your mail. I do not recommend using this method for more than a month now and then.
Professional mailing services are the best choice. Depending on how much mail you get, there are several levels provided by most services along with additional services such as registering vehicles for you, receiving legal notices, faxing, free internet in their office when in town, providing your location to friends and family with your permission, etc.
The best service for you depends on A) your residency choice, B) how much mail you get, C) price, D) extras provided with each plan, E) miscellaneous such as the wheel tax in some South Dakota counties which does not apply in others according to where your mail service is.