metric ton
吨 centers on a unit of weight: the metric ton. It is a modern character used specifically for this measurement.
吨 is a modern character created by combining 口 (mouth) and 屯, likely chosen for its sound to represent 'ton'. The exact historical development is uncertain, but it is a relatively recent coinage for the metric unit.
The left side 口 (mouth) and right side 屯 can be imagined as a mouth pronouncing 'ton' while looking at a heavy weight: 吨 is a metric ton.
For トン, the reading is exactly the English word 'ton'. Picture a heavy ton weight with the character 吨 stamped on it, and the sound 'ton' comes directly from the unit itself.