How long has he been parked there?
A Relative and other accounts here a couple or few years ago about Arizona RV Parks.
The authorities apparently were taking note of cars and RVs that had been there more than ten days, I think, were the reports. They came into the parks and were issuing tickets for not being registered or plated in the state as Residents.
In Az I think they personal property taxes and RV Registrations are not that cheap.
It would mess up your insurance, voting, and going back home and redoing everything and paying registration or taxes again there.
Don't remember reading of any outcome on the situation. It could put a serious crimp in RVParks incomes and in local groceries and other suppliers, if people started vacationing else where.
Taxes and more taxes is the theme today. Canadians have the six month rule to deal with as well as Residents of most or all states but Taxing Authorities, and Legislatures seem determined to make everyone stay home or pay though the nose.