Danny, Stu is right; it's mostly a matter of numbers. Campbell is a Highland clan, dominant in Argyll, and there are a few sons of Thom who stem from that region as well. There was apparently one Thomson at least, from ancient times, and a few MacTavishes. MacTavish is an anglicized version of the Gaelic name which also means son of Thom.
But statistically, the vast majority of Thom(p)sons were lowlanders, from nowhere near Argyll. And as Stu says, it's a patronymic, meaning anyone whose father was named Thom or Thomas could have taken the name as a family name at some point when it became popular to do so. So lots of Thomsons and Thompsons arose with no actual relation to each other. Only a few of them had any relation to Campbell or Argyll. But some of those few like to pretend that all these numerous Thomsons are part of their clan, and the people who sell clan crest jewelry and such have taken to using Campbell imagery in their products, fooling a lot of people into thinking they were related to Campbell. Many of these commercial outfits peddle a lot of claptrap in the name of family history.
So it's vaguely possible you are related to somebody who hung out with the Campbells, but it's a lot more likely your people came from the border areas. There are even a lot of Thompsons in Ireland, specifically in Country Antrim, where it's a common name. It takes research to find the exact connection.