You're in a tough spot. Storage unit businesses that do not provide their tenants with AC outlets inside the units are trying to tell them something: "don't use our electricity." Outlets are rare for most non-commercial units. Besides, most EMs after 1970 are 3-wire systems -- the third wire being ground -- which means the 2-wire hack suggested in post 5 can pose risk of electric shock. So even if you did have enough amps available from, say, a light fixture, you'd somehow have to work that ground wire, if it's accessible, into your hookup. Some businesses do provide common power outlets outside but that can involve multiple extension cords.