We are full-timers. We use Escapees RV Club mail service, and this provides a legal domicile address for us in Texas. Good Sam also provides this service for full-timers. These 2 clubs provide this service for just a few states. Check their websites for terms, conditions, various regulations, and other relevant information. The address reads as if it was an apartment. We use this address for vehicle registration, driver's licences, taxes, banks and credit cards, etc.
The service costs about $100 bucks a year, plus postage depending upon what you're having forwarded. Generally with packages/parcels I have them delivered to an RV park that we are or will going to.
Approximately every 2-3 weeks I call Escapees and have them forward our mail to a relevant RV park (either where we currently are, or going to be in the near future). I always check with the relevant RV park first to make sure I have the correct address, and that they will accept mail and package delivery, and as a general heads up/courtesy to them. Never had any problems, even when I've had mail delivered to a Canadian RV Park.