The Mustangs were outscored seven tries to six at Tarewa Park, but goal kicking proved crucial, as talented standoff Jahlon Governor nailed all six conversions as well as a penalty.
"We were up by a few, but same thing as every week," said coach Haze Reweti, who was still out with a hamstring injury but nonetheless appeared in full kit on the bench to keep Coastline guessing.
In very warm conditions, the Mustangs only had two actual reserves, so Reweti had to face the fearful prospect they would get run down late in the second half for the fourth straight game.
"It did cross my mind, but from the sideline, it looked like the boys were adamant we wouldn't lose.
"Luckily we got away to a big lead at the start."
Other key Boxon and Wanganui rugby players in the squad this campaign were Bernard Matthews, Brooklyn Herewini, Josh Fifita, Gordon Karaitiana and the inspiring captain Woody Martin, who moved from lock to the second row against Coastline.
"Tough as nails. He was his usual self, and the boys just backed him up," said Reweti of his skipper.
The win was of great relief to the Mustangs, who lost their opening game to the Wellington Orcas 36-22 after being deadlocked 22-all in the second-half, before gutting identical 24-22 defeats to the Northland Swords and Sharks when they scored tries right on fulltime.
The Swords went on to win the North Island title on Saturday when they scored again in the last minute to upset the Orcas 28-24, therefore earning the right to play South Island champions the Otago Whalers on October 5.
"It was a close comp, come next year the boys and myself will try to get behind Manawatu and see if we can get that [club competition] going," said Reweti.
"Rugby [union] enjoyed the boys this year, but we want to play league."
