Counties, cities and towns which shall or have issued bonds under authority of Article X, Section 35 of the Constitution shall levy a special tax payable annually in a total amount not to exceed five (5) mills on the dollar in addition to the legal rate permitted on the real and personal taxable property therein to provide for the payment of the principal and interest as they mature of all bonds heretofore or hereafter issued under authority of Section 35, Article X of the Constitution; provided, however, the county or municipality may, initially and from time to time thereafter, suspend the collection of such annual levy when not required for the payment of the principal of and interest on its bonds; and provided further that in event total net taxable valuation of the issuing county or municipality has declined from that in existence at time bonds were issued, and if necessary to assure payment of principal and interest on the bonds, a special tax up to five (5) mills shall be levied on the real and personal taxable property therein based on the issuer's total net taxable valuation in existence at time bonds were issued.