Update, picked up a 24 foot travel trailer for a friend today. Trailer weight was close to 6k. Doing 65-70 most of the way I got 11 mpg. I'm happy with those numbers. And obviously she performed well. I didn't know the TT was back there most of the times besides the road grade changes.
2012 F350 XLT 6.2L CC LB FX4