Sec. 5. If a plan also contemplates that sewage treatment for the district will be provided in cooperation with a municipality, the order must provide that:
(1) at least one (1) trustee shall be appointed by the executive of the municipality; and
(2) at least:
(A) one (1) trustee shall be appointed by the fiscal body; and
(B) one (1) trustee shall be appointed by the executive;
of the county having the largest amount of territory in the district.