You may be asking a bit much for 13mpg. Though with the 6.0L, you will probably get better. I get between 8.5 - 10mpg pulling my 5er, which is very similar to yours in weight, and I suspect design. As said above, it's that huge wall you're draggin in the wind more than the weight. But if you like the layout, that should be a bigger factor in your buying decision than the weight of the trailer.
Ed and Sharon
2010 Wildcat 28RKBS
2011 Ford F-250 XLT, 6.7L PSD
Ride-Rite Airbags and Airlift wireless remote compressor...nice!
I thought I was wrong once, but I was wrong!