Matt Roberts and Jean-Baptiste Bruziller crossed for the South's tries with outside-half Rhys Patchell adding a conversion and three penalties.
For the Dragons, who are still without a victory this season, there were two penalties, one each from outside half Tommy Haynes and replacement Josh Prosser.
The North eased to the top of the league with a comfortable bonus point victory over the Ospreys.
Owain Rhys (2), Chris Edwards, Thomas Pascoe, Jack Dando, Lee Gunter and Lewis Williams crossed for tries with Williams, who was captain for the night, adding a penalty and four conversions to finish with 16 points.
Nathan Brookes raced over for a brace of tries for the Ospreys whilst Andrew Claypole and Tom Cole added singles and Tom Carpenter kicked two conversions and a penalty.
North team manager Nigel Way said; "We raced into a 15-0 lead, but they pegged us back and made us work hard for the win. However, that's three victories so far and another bonus point too.
"The coaches, Lee Jarvis and Dale McIntosh, have already done a good job this season and they are continually rotating the squad to make sure each player gets his opportunity."
The Ospreys Under 18s defeated the Dragons 27-10 at Llandarcy with Tom Priday kicking 12 points.
Deeke Morse and Sam Hughes scored first-half tries as the Ospreys led 20-3 at the interval and Lloyd Pears made sure after the interval with his side's third try.
Openside flanker Gavin Jones scored a try for the Dragons on the hour whilst Adam Jones added a conversion and a first half penalty.