Radiator cooling isn't a problem. Ford tow package has allowed for extra cooling radiator and transmission cooler. Your problem will be towing a 6700lb. empty weight trailer + 6,000lb. Expy weight = 12,700lb combined weight against a 15,000lb GVWR. And you haven't even loaded the trailer! I am quoting my 2007 XLT specs for standard size Expy (not EL). And, not exceeding a 920lb. tongue weight. I opted for a Lance 28' trailer with 4,700lb empty weight, 6,300lb. gross weight. I drive with a weight no where near 6,300lbs. Empty holding tanks. Anything I can do to keep the weight down. I got rid of the "goosey" Expy "Soccer Mom" suspension and installed Rancho Quick Lift 7pt adjustable coil over shocks. Eliminates rear end squat. Installed Hellwig anti-sway bars front and back. Kicked up the HP with Gibson Side Swept Exhaust and K/N cold air filter. As a result, I have a solid truck feel suspension instead of the porpoise, bouncy stock suspension. I have towed my trailer many thousands of miles without an incident of overheating. The big issue is control of the trailer for a smoother, non-incident drive. Best of luck on your decisions.