I live in Grovetown, just east of Thomson. I have been on the Thomson Bypass (as it's signed) many times. It is indeed signed as US 78, GA 10, and GA 17 Bypass. According to former maps, US 78 (and I think GA 10) traveled through the city, on GA 17.
If I get a chance, I plan to drive the bypass this week to see what it's signed as.