In California’s notoriously expensive Bay Area, living out of one’s vehicle has increasing appeal. Recreational vehicle (RV) parks, once the domain of seasonal workers and adventurous retirees, now host local residents with full-time jobs aiming to cut costs — by avoiding high housing costs altogether. These days, RV parks are filling up even more with state residents aiming to save cash. Among them are brothers Stefan and Sebastian Goins, who recently made the leap to RV living after six years of rent hikes in their Pleasantville apartment. They drained their savings to buy a $24,000 32-foot long traveling trailer. But...