Am I stretching my ecoboost limits?

I wouldn't do it. I'm towing a 33' that is 7500lbs(loaded) and i wouldn't want to go heavier. The motor will have plenty of power to get you up the hills but it's coming down the other side that could get a little uncomfortable. You will have close to 10,000lbs pushing your 5700lb truck down the hill. The little motor pulls great but with 10,000lbs it will give you absolutely no help with engine breaking. You will find yourself building up speed going down steep grades and end up having to ride the breaks hoping they don't overheat by the time you get to the bottom. As capable as these trucks are i still think a trailer that size is 3/4 ton territory. Just my 2cents.