I tell people I get 10 mpg because that is what I average on a trip.
I have had days fighting cross winds were I have dropped to 7.5 mpg and I have had days when I get 11.5. My record for one day was 12.7 with an honest to goodness 25 mph sustained tail wind. That will never happen again.
We load the TT for every trip. Food, water typically 2/3 full, extra 5 gallons of gas and what ever gear we think we will need. Weight seems to have less affect that wind, hills and traffic. Most of my trips are 1000 to 1500 miles over two or three weeks, the average mpg is almost always about 10.