My experience towing in Cali is the truckers will haul buns if there are no cops around but do the limit if they're in the area. Let the truckers be your guide.
I feel the 55mph limit in Cali is ridiculous. To me, a 15+mph speed differential is dangerous with one lane doing 55 and the other lane doing 70+. Traffic should flow together, it should blend. Has the 55 limit proven itself to do anything safety related? Here in AZ, the limit is 75 for everyone with most doing 5 over. Does AZ have more safety related issues by allowing towed vehicle to go 20 mph faster than Cali? I haven't found any studies to say one way or the other.