I towed a 35 foot trailer with a 150 for 3 years. It did fine, I was at my limit with the payload and just under my tow capacity. The truck sat pretty level without air bags or anything as I had the equil i zer hitch doing what it was suppose to. I was happy with the set up. If it was not for other life events I would still be towing that way.
As for speed, I try to keep it under 65 at all times. First for tires and second for gas. I set the cruise control at 62/63 generally and that is ok gas milage, if I hit 65 or higher I can see the gas gauge move.