I've always noticed drag is the biggest factor as well. I used to have a chevy s10 with a 4 cylinder and 5 speed manual. I towed a small empty open utility trailer with gate(5'x8' or so) about 60 miles down the highway, the truck wouldn't go past about 60-65mph at full pedal in 5th gear. The trailer probably only weighed about 1,000lb.
On the return trip with the trailer loaded(just say it now weighs 1,500-2,000lb) it was more aerodynamic and I was able to fairly easily hold 70mph the whole trip.
The following trips I removed the gate altogether and it was a drastic difference.
This proves to me how much drag effects mpg, pulling power, etc.
My ram with over 1,000lb in the bed got 20mpg avg on a trip from florida to arkansas and back last summer. This spring I averaged 12mpg on a trip with a 4,000lb(loaded) enclosed trailer to NC and back. I'd bet most of the drop in mpg is from the drag, not the weight.
Breathing in and out is good enough for me.