Minnesota Rules 7413.0700 discusses failure to maintain insurance. Here it states that if the MN Department of Public Safety records indicate that any owner has ceased to maintain a required insurance policy, the commissioner shall suspend the registration certificate and license plates for the motor vehicle or motorcycle and may demand their immediate surrender by the owner.

Minnesota Statute 169.798 also discusses insurance under the title of Rules of Commissioner of Public Safety. Here it states that every owner, when applying for motor vehicle or motorcycle registration, registration, or transfer of ownership, must attest that the motor vehicle or motorcycle is covered by an insurance policy.

So if you want your vehicles to remain registered in Minnesota then they need to maintain at least the state minimum liability insurance on them. To find out if there is a way to lower insurance or "suspend" registration on your vehicles while they are in storage and not being used contact the MN Driver and Vehicle Services.