Our B+ did pretty well with it's first trip towing the car trailer. It's about 18,700 #s total and on the way up it would pull most hills in Overdrive if i'd hit the bottom of them at 75 MPH. Very comfortable cruise up. I estimate it got around 7.5 mpg on the way up with me not being conservative with the go pedal at all. It typically gets 8.5 mpg without a trailer.
Yesterday coming home was a different story. Huge headwind that was sometimes a cross wind as well, 30-40 mph wind the whole 2.5 hours home. I'm guessing I was getting below 5 mpg and had two fistfuls of steering wheel the whole way home. It's no fun when you rig moves over a half lane and there is nothing you can do about it.
It would still run 60-70 MPH depending on conditions, but I used to haul mode and spent most time closer to 60 mph. The trailer wagging really wasn't an issue...in fact I pretty much didn't know the trailer was back there. It was mostly just trying to keep the 11.5 foot tall rig in my own lane. My friend with a Ram 1 ton dually pulling a toy hauler usually gets around 13 mpg pulling averaged 8.5 mpg coming home the same route. It was brutal.
Sometimes you have to do what you have to do to have fun at the track though. Still worth it...