The next maintenance-due dates can be added into the resource details. This is shown when viewing the resource details, as well as via the maintenance report.
The maintenance report is available via the 'Resources' menu, within the 'Other reports' link. This report lists all resources in the selected group, along with their next-service dates, number of days till it's next service is due, etc.
When a 'contact' person has been set in the resource details, an action item is automatically created roughly a week prior to the service-due date. When the resource (eg. a vehicle / radio / etc) is serviced, the action item should be marked as completed, and the 'maintenance due date' should be updated.