From the beet fields of North Dakota to the National Forest campgrounds of California to Amazon's CamperForce program in Texas, employers have discovered a new, low-cost labor pool, made up largely of transient older Americans. Finding that social security comes up short, often underwater on mortgages, these invisible casualties of the Great Recession have taken to the road by the tens of thousands in late-model RVs, travel trailers, and vans, forming...