Every state department or state officer, upon request of the department, shall furnish any service or assistance in the investigation of the need or feasibility of all or any part of such general or coordinated plan and the cost of construction, operation, and maintenance thereof, of the financing of construction and rates or returns that may be required to operate and maintain all or any part of the plan, of the amortization of bonded or other indebtedness that may be placed on all or any part of the plan for the cost of construction thereof, and shall render any other service which the department deems necessary for the maintenance of any priority in the State for the purposes of all or any part or unit of the plan and the future development and completion of it in the public interest.