Sheryl,
As far as insurance for the vehicles, your tenants need to show that they have liability insurance in case they damage your property. This should be a requirement in your lease. You want to protect your business from careless (and not so careless) accidents.
As for the property of the tenant that is stored in your facility, many companies require insurance in their lease. At the very least they have a limit of what can be stored (usually $5,000 worth of property).
Many operators offer their clients insurance during the leasing process. The options vary but the tenant can pay the premium along with their rent or they can mail in a brochure to the insurance company.
Offering insurance can protect you from a tenant claim. It makes sure that they understand that, as a landlord, you are not responsible for their property.
There are several companies that offer either program. Look on the SSA website or in the ISS magazine for details.
Toni