Ask any OBS Ford owner what the secret is to their 120 hp 300 I6 towing 27,894 lbs uphill with crushed glass in the crankcase for oil?
At any rate, seems like you're not asking for a one time, down the road deal, but more like a, I want to live the Hot Shot life and do this down the highway all day deal.
In which case, you not only need a bigger truck, you need a way newer one.
If I were towing 15k, everyday, on freeways, through cities, for a living, I wouldn't do it with anything less than a 2015 or so diesel 1 ton, whatever brand tickles your fancy.
If you're just saying 11-15k because it sounds cool, or seems like more than you need, take a realistic look at what you're really towing and how often. My 18', all steel car trailer weighs 1860 lbs, and even my diesel 4x4 4runner is under 6k. Any full size truck ever made will easily tow my trailer and any one of my other trucks.