Can't help you with the trailer brake question - but in terms of MPG you'll get 5 to 10. 5 into the wind, 10 with a huge wind behind you, and average 7.5. Those numbers are pretty good for any diesel truck combination pulling a 13k or better 5th wheel at 65 mph. Just no good way to get past the average of 7 or 8, especially with the 4:30. Some get a little more, some a little less, but most hit these averages.
Should pull really well though, plenty of torque.
Let us know what you get when you hook it up. What is the brake question, BTW?