The only answer for that is that it depends: Dry road or wet road, fresh driver or tired, a steady wind on a bridge is going to affect you differently than the same on bald prairie vs. a highway with a tree belt or mountain valley. Gust are simply unpredictable - see An RV crash to remember - YouTube
That being said a proper setup goes a long way to minimize the negative effects of towing.
I feel very little with my setup running in steady winds at 50km/h (30mph) with gusts to 60km/h (40mph) & I have been caught in winds with gusts at 80 + km/h (50+mph). I can say that i really prefer calm winds when towing or a light tail wind.