I've used USPS General Delivery for mail forwarding. Just be sure to use a small town Post Office away from major urban areas. Big city General Delivery usually goes to the main PO down town which could be a royal PITA to travel too/park with one's truck/camper rig.
The down side to USPS. The agency will only forward mail for 30 days. As such, I usually cancel before the 30 day deadline and reestablish a new forwarding address to General Delivery.
Typically I setup mail forwarding to General Delivery for a 7-10 day period to a PO along my tentative route. I will setup a new PO before arriving at the previous forwarding to PO. That way, there is generally not a lapse in forwarding. Further, with 7-10 days to bounce around between POs, I'm not locked into a travel itinerary and can easily deviate along my general direction of travel.
It's not pain free or 100% reliable, but, the best I've found without paying a third party company a premium to forward mail.
I have opted out of all the junk mail and solicitations I possibly can. You don't want unsolicited credit card offers with your name and address sitting around POs across the country. Best not to have any important paperwork going through the forwarding process to General Delivery.
On a side note...
All of my banking, insurance, investing/brokerage is done paperless, online. These items are my biggest concern and paperless is definitely the way to go. You don't want statements sitting around in some PO. Individuals bent on fraud, while not able to hack an account simply from a statement, can eventually data mine to the point they may be able to hack a financial account. Why it's imperative to use robust online security and at a minimum, two step authentication to access these accounts.