It shall be the mandatory duty of any common sewer district which shall issue revenue bonds pursuant to sections 204.250 to 204.470:
(1) To fix and maintain rates and make and collect charges for the use and services of the system, for the benefit of which revenue bonds were issued, sufficient to pay the cost of maintenance and operation thereof;
(2) To pay the principal of and the interest on all revenue bonds issued by the common sewer district chargeable to the revenues of the system; and
(3) To provide funds ample to meet all valid and reasonable requirements of the resolution by which the revenue bonds have been issued.