In GA the published speed limit for towing is 55. I have never seen a sign saying such, but I know that the limit exists. That being said, I don't think anyone follows it. I have been passed when I was towing and driving around 60mph, by so many different combinations, that I feel like I am creeping, but I feel more comfortable and in control.
Last weekend and this past Friday, I spent a number of hours on interstates in the state and it seemed NO ONE drove 55. Friday, I was in rain on I75 and the only place people were driving 55 was where traffic was backed up due to accident and all lanes were at about 20mph.