Being a RV forum, the emphasis is on RV storage. In the real world, somewhere around 50% of the usage will be RV storage in those complexes that are successful. There are innumerable reasons for people seeking this type of storage and will expand the market to make the project feasible. There simply are not enough people with a RV that are willing or able to pay the rent you will need.
In Iowa, the basics you will need are heat, abundant lighting, heavy insulation,30AMP service, individual meters and central bathroom facilities. Local zoning laws will control other items of necessity. Fifty-foot, forty-five, and forty foot units will work best for length. Units between fifteen wide and twenty wide work best and will be somewhat dictated by the land available and the layout of the complex.
You will have taxes to pay, insurance, maintenance, and man hours to manage the facility. All-in, they can be built in your location for $35-$45 per square foot and will need to be rented in the $300 per month range to make a profit. Most any city of 100,000 population will support a limited number of these units if properly built and managed.
