I'd add these two resources. I once read in one of these of a few states that are probably the best for those living out of their vehicle to register in.
You may want to check your Libraries for these:
"Roll Your Own: Complete Guide to Living in a Truck, Bus, Van or Camper" (1974)
http://www.amazon.com/Roll-Your-Own-Complete-Living/dp/0020810504
"Rolling Homes: Handmade Houses on Wheels" (1979)
http://www.amazon.com/Rolling-Homes-Handmade-Houses-Wheels/dp/089104129X
i know these are a bit dated, but there is still some ideas represented in these that may be valuable to a
newbie.
I have actually seen people put up Rural Mail Boxes on the side of secondary roads and leave the red flag
up with a note in it to the delivery person stating that they have bought property and would like a Box Number assigned. They of course leave their name. Usually the next day there is a reply with a number
written on a form to mark on the box. Then the person prior to leaving town leaves a note to hold their
mail at the PO for them. They will pick it up on return. Then a small lock is placed on the box above the
letter slot such that loads of junk mail and ad's cannot be crammed into it. They won't get much mail otherwise.
But there are ways of getting this stuff. Mail Forwarding Services used to be common for this too. If you know some Truck Drivers, they seem to know all the tricks.