Did a Ford salesman tell you that the Ford was the way to go? A Ram salesman would tell you a RAM 3500 would best.
Really, all 3 of the makers diesel trucks seem to be doing pretty well at this point. I'd drive them all before making such a major purpose. You will find differences in step-in height, handling, ride, cab noise, etc. that may be important.
For that much weight (both trailer and family) I think a dually is the way to go for sure. Weight adds up fast and the extra wheels, in my experience, make a tremendous difference in how the truck rides and drives.
The Ford 6.4 would not be my first choice for sure. They are known to get rather poor fuel economy compared to other engines and are very complicated. If you stick with Ford I would only get the 6.7.
I've had Chevy's personally and the company I work for has had Fords. My Chevy's have given me nearly flawless performance. The Ford 6.0 trucks at work have been terrible. Not just the engines but I much prefer the Chevy's ride and handling.