Wind resistance is 4 times as great when going 60 as compared to 30.One additional thing to consider . . . wind resistance !
My experience with towing various boats is that they pull very easily for their size and offer very little wind resistance. When you hook up a cargo trailer or a toy hauler and hit the interstate, I think the wind resistance is going to surprise you. The faster you go the more you will notice the resistance . . . and its influence on your gas mileage. I hope you find a small toy hauler.