I normally average 14-15 mpg when hauling my camper, or pulling our big ol' Sundowner horse trailer. However, a few weeks ago we went to Navasota and hauled my new draft horse back home, it was a bit over 200 miles each way. On the way down, my calculations show that I got about 14 mpg, but on the way back it was 9. 8

:{ . The horse weighs about 2,000 pounds but shouldn't make that big of a difference. We had a stiff headwind, but nothing outrageous. I had filled up in Navasota, and I noticed that the truck seemed to have less power than normal, no signs of clutch slippage or anything obvious though. I filled back up in Ft. Worth and once again was back to the usual 20 mpg range.
If your truck has pretty consistent mileage unloaded, it is easy to figure out your mileage when driving loaded even for a partial tank. Just keep track of how many miles you drove loaded versus unloaded, you can use your average to figure out how many gallons were used driving unloaded and use the remainder to calculate the loaded MPG.
My truck has been very consistent in unloaded mpg, it has a standard deviation of only 0. 8 mpg in a couple of hundred fillups. Here's a chart from Excel showing the mpg for unloaded fill-ups dating back to 2001, about 115 tanks worth. "Unloaded" meaning no camper or horsetrailer, but often I am hauling stuff or pulling small trailer.