With transliterations, it should be normal to have variations. In Taiwan, we say dun4 in Mandarin. Most people say tōn in Taiwanese, a sound from Japanese indeed.
tōn is 噸 spoken in Taiwanese here in Taiwan. When there's a number before it, it's used this way. Like in 1.5 tōn. But when followed by other words or the word before is not a number, it's usually tùn as in tùn-ūi and kong-tùn.