Using a campground address might work but if you're moving between campgrounds and not staying year-round, you still need a domicile address that can be used for everything year-round. Would the campground address handle your mail even when you move to another campground? You need an address for vehicle registrations, licenses, voter's record, medical insurance, Medicare, credit cards, etc. It should be one address used all the time for everything. That's where a mail service comes into play. It's your permanent address even if you move around.