Even if you live 100 percent in an RV in the US, you still have to establish residency in one state or another for tax purposes, and for a place to register your RV, get a driver's license. get health insurance, register to vote, etc. You need an actual fixed location for that address; mail drops and PO boxes don't work. If you're a full time RVer in the US, you still need to have an actual street address; I imagine lots of them use a friend's/relative's home address in their home state for things like their driver's license, or else they choose a state to establish residency in, and stay there long enough to meet the criteria for residency.
"No matter where you go, there you are."