Former van-denizen here. I always used friends' addresses (with their permission, for a limited amount of time) when I needed to get important docs or plates. Usually it's not too hard to find someone to let you receive some mail at their house. Utility bills were always a problem for me, but there are usually ways around. Proof of filing a tax return in-state is enough in most cases, even if you're not "at" that address anymore.