How much reduction in fuel mileage can I expect pulling a 6x12 cargo trailer?
The added weight in the trailer will be less than 1000 lbs.
4 to 5 mpg lower?
I want to figure out the additional cost to pull the trailer 3800 miles

When I pull my snowmobile trailer 8' wide and 5' tall frontal area and about 1400 lbs loaded, I get about 17 MPG at 65 MPH. I get about 15 at 75 MPH. These are number I just got when I made a 800 mile round trip to Utah. I run 265 tires so this might have some affect and I also run my Juice on level 2. I genrerally get 2-3 MPG better when I'm empty.

That's not much weight so I wouldn't expect a significant drop in MPG's with a standard height cargo trailer. JJ :)

For comparison.

I just towed my 5th from SF Bay area to Billings, MT and averaged 10.2.

That was an average speed of about 72mph.

I cannot complain.

The difference in reduced fuel mileage has as much to do with the weight of the right foot as it dees the mass being moved. Results vary widely based on what you are racing against.

RF.