You have three choices:
1. Get a trusted neighbor, relative or friend to pick up your mail from your box, sort out the junk, and forward the rest to you to an address you specify.
2. Go to a UPS store or similar and hire a Private Mail Box (PMB). Then change all of your accounts ( banks, credit cards, medical, IRS, Social Security, everything in your life) to the new address. Then call the store and have them mail the box contents to you.
3. Similar to #2 above. Go to a mail forwarding company that caters to full time RVers. Google mail forwarding to find them.
If you chose #2 or #3 above, you will considered as "moving" as far as your credit report, and other aspects of your life. When you return from your trip, if you go back to your previous mail handling, you will have to "move" again back to your previous address.
The Post Office will not hold your mail and then send it to multiple address as you travel about the country.