I too had a 2008 ford diesel, mileage on it was 5-9 never double digits. so I bit the bullet and got the 2011 F350 srw crew cab 4x4, I pull a 365 road warrior which is also 41' just made a trip of about 1300 miles and averaged 8.3 mpg at about 77 mph. prior to that I made a 2600 mile trip pulling a 8.5 x 20 enclosed trailer with 6 bikes and a bunch of other equipment and averaged 10 doing between 75 and 80. currently I have about 4400 miles so I do assume it will do better as it gets broke in, also, around town empty I'm getting 15-17 mpg which is a huge improvement over the 08. to sit in the truck it's not a whole lot different than my 08, but what is noticeable is the motor and tranny, when you step on it, hold on it accelerates like a strong running gas motor, if that makes any sense, no turbo lag. the tranny always seems to be in the right gear and it will pull a steep grade with no problem. The exhaust brake is automatic, so you have to be tow haul mode for it to work, but coming off a pretty steep grade no problem.
so was it worth it? obviously I couldn't justify it on fuel mileage, but at the end of the day it was worth it, I went from a f-250 to a f-350 and it handles the trailer really well, the sway control is pretty awesome as well, in high wind situations the rv doesn't even move. Many people are going for the rebates on the 2010 6.4's, and after having the 6.7 there is no comparison to my 08 6.4