From the beet fields of North Dakota to the wilderness campgrounds of California to an Amazon warehouse in Texas, people who once might have kicked back to enjoy their sunset years are hard at work. Underwater on mortgages or finding that Social Security comes up short, they're hitting the road in astonishing numbers, forming a new community of nomads: RV and van-dwelling migrant laborers, or "workampers." Building on her groundbreaking Harper's cover...