I would think that fuel cost would be one of your major cost areas to attempt to control. In pulling my 5th wheel trailer, which is pretty tall and fairly flat on its front face, fuel mileage can vary a great deal depending on the wind and it's direction relative to my travel. I've gotten as high as 12 and as low as 6, with the 6 being directly into a 30 MPH headwind, even though I was only going 55. That's still 85 combined and you know what your mileage is like going 85.
I think you need to look carefully at your setup and see if you can't reduce the wind resistance of your trailer. All big-rig trucks use deflectors, and you could maybe do some streamlining of your trailer to help the air flow over the front. And I'm aware that there are some 'tabs' (don't know exactly what they're called) that can be attached to the rear of the trailer's sidewalls that stop the air from causing so much turbulence behind the trailer.
Another thing I would do RIGHT NOW is to check the level of the gear lube in the rear axle. GM is known for under-filling these, mine was about 1 qt low. You don't want your diff to have too little lube, so check it. Only takes a few minutes.
2008 2500HD LT CCLB/Ally. Line-X, B&W Companion, MaxLoader Springs, Edge Insight CTS, Kennedy Custom ECM
2009 Arctic Fox 5er, Silver Fox Edition 29-5T, 3.6 Onan LPG, SteadyFast Stabilizers